Improve Doxygen docs for nested namespaces

Improve Doxygen docs for nested namespaces

	* doc/doxygen/doxygroups.cc (std::tr2, std::__gnu_cxx): Improve docs.
	(std::experimental): Add docs.
	* doc/doxygen/user.cfg.in (PREDEFINED): Expand macros for __cxx11
	namespace to nothing when generating docs.
	* include/bits/regex_constants.h (std::regex_constants): Improve docs.
	* include/std/chrono (std::chrono): Likewise.
	* include/std/functional (std::placeholders): Likewise.
	* include/std/thread (std::this_thread): Likewise.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@270805 138bc75d-0d04-0410-961f-82ee72b054a4