doc: Ensure documentation lists output argument when it exists for all m-files. For new users of Octave it is best to show explicit calling forms in the documentation and to show a return argument when it exists. * bp-table.cc, shift.m, accumarray.m, accumdim.m, bincoeff.m, bitcmp.m, bitget.m, bitset.m, blkdiag.m, celldisp.m, cplxpair.m, dblquad.m, flip.m, fliplr.m, flipud.m, idivide.m, int2str.m, interpft.m, logspace.m, num2str.m, polyarea.m, postpad.m, prepad.m, randi.m, repmat.m, rng.m, rot90.m, rotdim.m, structfun.m, triplequad.m, uibuttongroup.m, uicontrol.m, uipanel.m, uipushtool.m, uitoggletool.m, uitoolbar.m, waitforbuttonpress.m, help.m, __additional_help_message__.m, hsv.m, im2double.m, im2frame.m, javachk.m, usejava.m, argnames.m, char.m, formula.m, inline.m, __vectorize__.m, findstr.m, flipdim.m, strmatch.m, vectorize.m, commutation_matrix.m, cond.m, cross.m, duplication_matrix.m, expm.m, orth.m, rank.m, rref.m, trace.m, vech.m, cast.m, compare_versions.m, delete.m, dir.m, fileattrib.m, grabcode.m, gunzip.m, inputname.m, license.m, list_primes.m, ls.m, mexext.m, movefile.m, namelengthmax.m, nargoutchk.m, nthargout.m, substruct.m, swapbytes.m, ver.m, verLessThan.m, what.m, fminunc.m, fsolve.m, fzero.m, optimget.m, __fdjac__.m, matlabroot.m, savepath.m, campos.m, camroll.m, camtarget.m, camup.m, camva.m, camzoom.m, clabel.m, diffuse.m, legend.m, orient.m, rticks.m, specular.m, thetaticks.m, xlim.m, xtickangle.m, xticklabels.m, xticks.m, ylim.m, ytickangle.m, yticklabels.m, yticks.m, zlim.m, ztickangle.m, zticklabels.m, zticks.m, ellipsoid.m, isocolors.m, isonormals.m, stairs.m, surfnorm.m, __actual_axis_position__.m, __pltopt__.m, close.m, graphics_toolkit.m, pan.m, print.m, printd.m, __ghostscript__.m, __gnuplot_print__.m, __opengl_print__.m, rotate3d.m, subplot.m, zoom.m, compan.m, conv.m, poly.m, polyaffine.m, polyder.m, polyint.m, polyout.m, polyreduce.m, polyvalm.m, roots.m, prefdir.m, prefsfile.m, profexplore.m, profexport.m, profshow.m, powerset.m, unique.m, arch_rnd.m, arma_rnd.m, autoreg_matrix.m, bartlett.m, blackman.m, detrend.m, durbinlevinson.m, fftconv.m, fftfilt.m, fftshift.m, fractdiff.m, hamming.m, hanning.m, hurst.m, ifftshift.m, rectangle_lw.m, rectangle_sw.m, triangle_lw.m, sinc.m, sinetone.m, sinewave.m, spectral_adf.m, spectral_xdf.m, spencer.m, ilu.m, __sprand__.m, sprand.m, sprandn.m, sprandsym.m, treelayout.m, beta.m, betainc.m, betaincinv.m, betaln.m, cosint.m, expint.m, factorial.m, gammainc.m, gammaincinv.m, lcm.m, nthroot.m, perms.m, reallog.m, realpow.m, realsqrt.m, sinint.m, hadamard.m, hankel.m, hilb.m, invhilb.m, magic.m, pascal.m, rosser.m, toeplitz.m, vander.m, wilkinson.m, center.m, corr.m, cov.m, discrete_cdf.m, discrete_inv.m, discrete_pdf.m, discrete_rnd.m, empirical_cdf.m, empirical_inv.m, empirical_pdf.m, empirical_rnd.m, kendall.m, kurtosis.m, mad.m, mean.m, meansq.m, median.m, mode.m, moment.m, range.m, ranks.m, run_count.m, skewness.m, spearman.m, statistics.m, std.m, base2dec.m, bin2dec.m, blanks.m, cstrcat.m, deblank.m, dec2base.m, dec2bin.m, dec2hex.m, hex2dec.m, index.m, regexptranslate.m, rindex.m, strcat.m, strjust.m, strtrim.m, strtrunc.m, substr.m, untabify.m, __have_feature__.m, __prog_output_assert__.m, __run_test_suite__.m, example.m, fail.m, asctime.m, calendar.m, ctime.m, date.m, etime.m: Add return arguments to @deftypefn macros where they were missing. Rename variables in functions (particularly generic "retval") to match documentation. Rename some return variables for (hopefully) better clarity (e.g., 'ax' to 'hax' to indicate it is a graphics handle to an axes object).

########################################################################
##
## Copyright (C) 1996-2022 The Octave Project Developers
##
## See the file COPYRIGHT.md in the top-level directory of this
## distribution or <https://octave.org/copyright/>.
##
## This file is part of Octave.
##
## Octave is free software: you can redistribute it and/or modify it
## under the terms of the GNU General Public License as published by
## the Free Software Foundation, either version 3 of the License, or
## (at your option) any later version.
##
## Octave is distributed in the hope that it will be useful, but
## WITHOUT ANY WARRANTY; without even the implied warranty of
## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
## GNU General Public License for more details.
##
## You should have received a copy of the GNU General Public License
## along with Octave; see the file COPYING.  If not, see
## <https://www.gnu.org/licenses/>.
##
########################################################################

## -*- texinfo -*-
## @deftypefn  {} {@var{str} =} substr (@var{s}, @var{offset})
## @deftypefnx {} {} substr (@var{s}, @var{offset}, @var{len})
## Return the substring of @var{s} which starts at character number
## @var{offset} and is @var{len} characters long.
##
## Position numbering for offsets begins with 1.  If @var{offset} is negative,
## extraction starts that far from the end of the string.
##
## If @var{len} is omitted, the substring extends to the end of @var{s}.  A
## negative value for @var{len} extracts to within @var{len} characters of
## the end of the string
##
## Examples:
##
## @example
## @group
## substr ("This is a test string", 6, 9)
##      @result{} "is a test"
## substr ("This is a test string", -11)
##      @result{} "test string"
## substr ("This is a test string", -11, -7)
##      @result{} "test"
## @end group
## @end example
##
## This function is patterned after the equivalent function in Perl.
## @end deftypefn

function str = substr (s, offset, len)

  if (nargin < 2)
    print_usage ();
  endif

  if (! ischar (s))
    error ("substr: S must be a string or string array");
  elseif (! isscalar (offset) || (nargin == 3 && ! isscalar (len)))
    error ("substr: OFFSET and LEN must be scalar integers");
  endif

  offset = fix (offset);
  nc = columns (s);
  if (abs (offset) > nc || offset == 0)
    error ("substr: OFFSET = %d out of range", offset);
  endif

  if (offset <= 0)
    offset += nc + 1;
  endif

  if (nargin == 2)
    eos = nc;
  else
    len = fix (len);
    if (len < 0)
      eos = nc + len;
    else
      eos = offset + len - 1;
    endif
  endif

  if (eos > nc)
    error ("substr: length LEN = %d out of range", len);
  elseif (offset > eos && len != 0)
    error ("substr: No overlap with chosen values of OFFSET and LEN");
  endif

  str = s(:, offset:eos);

endfunction


%!assert (substr ("This is a test string", 6, 9), "is a test")
%!assert (substr ("This is a test string", -11), "test string")
%!assert (substr ("This is a test string", -11, 4), "test")
%!assert (substr ("This is a test string", -11, -7), "test")
%!assert (substr ("This is a test string", 1, -7), "This is a test")
%!assert (isempty (substr ("This is a test string", 1, 0)))

## Test input validation
%!error <Invalid call> substr ()
%!error substr ("foo", 2, 3, 4)
%!error substr (ones (5, 1), 1, 1)
%!error substr ("foo", ones (2,2))
%!error substr ("foo", 1, ones (2,2))
%!error substr ("foo", 0)
%!error substr ("foo", 5)
%!error substr ("foo", 1, 5)
%!error substr ("foo", -1, 5)
%!error substr ("foo", 2, -5)