PR 92830 reports that we always use "gcc/Warning-Options.html" when we
emit escaped documentation URLs when printing "[-Wname-of-option]" for
a warning.

This page is wrong for most Fortran warnings, and for analyzer warnings.

I considered various schemes involving adding extra tags to the .opt
format to capture where options are documented, but for now this patch
fixes the issue by introducing some special-casing logic.
It only fixes the URLs for warning options, not for other command-line
options, but those are the only options for which get_option_url is
currently called.

gcc/ChangeLog:
	PR 92830
	* configure.ac (DOCUMENTATION_ROOT_URL): Drop trailing "gcc/" from
	default value, so that it can by supplied by get_option_html_page.
	* configure: Regenerate.
	* opts.c: Include "selftest.h".
	(get_option_html_page): New function.
	(get_option_url): Use it.  Reformat to place comments next to the
	expressions they refer to.
	(selftest::test_get_option_html_page): New.
	(selftest::opts_c_tests): New.
	* selftest-run-tests.c (selftest::run_tests): Call
	selftest::opts_c_tests.
	* selftest.h (selftest::opts_c_tests): New decl.