doc: Refine optimset, optimget docstring on paramter matching. (bug #64525) * optimget.m, optimset.m: Clarify that pattern matching is case insensitive and works by matching characters at the start of the parameter name. Remove double negatives from notes about ignoring non-standard options. Ensure consistent language used for options/parameters in both functions.

%!function out = bug54490 ()
%!  global k;
%!  k = 1;
%!  out = 3;
%!endfunction

%!test <*54490>
%! global k;
%! k = 2;
%! a = [5, 6];
%! a(k) = bug54490 ();
%! assert (a, [5, 3]);
%! k = 2;
%! a = [5, 6];
%! [a(k)] = bug54490 ();
%! assert (a, [5, 3]);
%! clear -global k;  # cleanup after test