libstdc++/doc: Document generate_canonical and variant compat macros.

libstdc++/doc: Document generate_canonical and variant compat macros.

The _GLIBCXX_USE_OLD_GENERATE_CANONICAL was introduced by
r16-6177-g866bc8a9214b1d that implemented P0952R2 [1] resolution
for LWG2524 as DR against C++20.

The _GLIBCXX_USE_VARIANT_CXX17_OLD_ABI was introduced by
r16-6301-gb3c167b61fd75f that resovled PR112591.

[1] https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/p0952r2.html

libstdc++-v3/ChangeLog:

	* doc/html/manual/using_macros.html: Regenerate.
	* doc/xml/manual/using.xml: Add entries for
	_GLIBCXX_USE_OLD_GENERATE_CANONICAL and
	_GLIBCXX_USE_VARIANT_CXX17_OLD_ABI.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>