libstdc++: Improve doxygen docs for <mutex>

libstdc++: Improve doxygen docs for <mutex>

libstdc++-v3/ChangeLog:

	* doc/doxygen/user.cfg.in (PREDEFINED): Define
	_GTHREAD_USE_MUTEX_TIMEDLOCK macro.
	* include/bits/std_mutex.h (mutex, lock_guard): Use @since and
	@headerfile.
	* include/bits/unique_lock.h (unique_lock): Likewise.
	* include/std/mutex (recursive_mutex, timed_mutex)
	(recursive_timed_mutex, scoped_lock): Likewise.

(cherry picked from commit b584cbdb0829bb0ad4e6ed7488a304ae4e76b870)