doc: Ensure documentation lists output argument when it exists for functions in libinterp/ 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. * __ftp__.cc, __magick_read__.cc, __pchip_deriv__.cc, bitfcns.cc, bsxfun.cc, call-stack.cc, cellfun.cc, chol.cc, conv2.cc, data.cc, debug.cc, defaults.cc, det.cc, dirfns.cc, display.cc, dot.cc, error.cc, event-manager.cc, fft.cc, fft2.cc, fftn.cc, file-io.cc, getgrent.cc, getpwent.cc, getrusage.cc, graphics.cc, hash.cc, help.cc, input.cc, interpreter.cc, kron.cc, load-path.cc, mappers.cc, max.cc, nproc.cc, oct-hist.cc, pager.cc, pinv.cc, psi.cc, rand.cc, settings.cc, sighandlers.cc, stream-euler.cc, strfns.cc, symtab.cc, syscalls.cc, sysdep.cc, time.cc, toplev.cc, utils.cc, variables.cc, __fltk_uigetfile__.cc, audiodevinfo.cc, audioread.cc, fftw.cc, ov-bool-mat.cc, ov-cell.cc, ov-class.cc, ov-classdef.cc, ov-fcn-handle.cc, ov-java.cc, ov-struct.cc, ov-typeinfo.cc, ov-usr-fcn.cc, ov.cc, octave.cc, profiler.cc: Add return arguments to @deftypefn macros where they were missing. Attempt to use standard naming convention for return variables. Occasionally improved the docstring itself by re-wording or adding code examples.