libstdc++: Minor fixes to doxygen comments

libstdc++-v3/ChangeLog:

	* include/bits/uses_allocator.h: Add missing @file comment.
	* include/bits/regex.tcc: Remove stray doxygen comments.
	* include/experimental/memory_resource: Likewise.
	* include/std/bit: Tweak doxygen @cond comments.
	* include/std/expected: Likewise.
	* include/std/numbers: Likewise.