diagnostics: get_option_html_page fixes

diagnostics: get_option_html_page fixes

While testing the --with-documentation-root-url= changes, I run into
[Wreturn-type] URL pointing to gfortran documentation where it obviously
isn't documented.  The following patch updates the list of options to match
reality (on the other side -Wconversion-extra is gfortran only option
documented in gfortran.texi).

Or, perhaps better use the attached patch instead, which doesn't have a
hardcoded list and instead uses the flags?  I went through options.c
and the updated list of options matches exactly the cases where CL_Fortran
is set for "-W*" options together with CL_C and/or CL_CXX (ok, there is also
-Wall and -Wextra, but hopefully we don't emit [Wall] or [Wextra] for
anything).

2020-05-04  Jakub Jelinek  <jakub@redhat.com>

	* opts.c (get_option_html_page): Instead of hardcoding a list of
	options common between C/C++ and Fortran only use gfortran/
	documentation for warnings that have CL_Fortran set but not
	CL_C or CL_CXX.