simplify rules for building DOCSTRING files (bug #47745) * libinterp/module.mk, scripts/module.mk: Don't create separate .DOCSTRINGS file. Create DOCSTRINGS files in $(srcdir). Use move-if-change to avoid rebuilding docs in tarball distribution.

%!function r = f (x)
%!  if (ischar (x))
%!    r = x;
%!  else
%!    error ("expecting character string");
%!  endif
%!endfunction

%!assert (eval ("f 10;"), "10")