lookfor.m: Overhaul function and fix bug #44924. * lookfor.m: Rewrite docstring. Match variable names in function prototype to docstring. Use more meaningful variable names throughout function. Don't continue to process a function if get_first_help_sentence returns an error (bug #44924). Iterate directly over cell list rather than using a for loop with a range to index into cell list.

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

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