libstdc++: Fix Doxygen warnings

libstdc++: Fix Doxygen warnings

libstdc++-v3/ChangeLog:

	* include/bits/alloc_traits.h: Use markdown for code font.
	* include/bits/basic_string.h: Fix @param names.
	* include/bits/max_size_type.h: Remove period after @file.
	* include/bits/regex.h: Fix duplicate @retval names, and rename.
	* include/ext/pb_ds/detail/priority_queue_base_dispatch.hpp: Add
	group open to match existing group close.
	* include/ext/pb_ds/priority_queue.hpp: Add blank line before group
	open.