libstdc++: Enable doxygen processing for C++20 components

Improve grouping, add @since and @deprecated information.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	* doc/doxygen/user.cfg.in (PREDEFINED): Enable doxygen
	processing for C++20 components and components that depend on
	compiler features.
	* include/bits/stl_algo.h (random_shuffle): Use @deprecated.
	* include/std/type_traits: Improve doxygen comments for C++20
	traits.