libstdc++: Improvements to Doxygen markup

libstdc++: Improvements to Doxygen markup

This attempst to improve the doxygen output to work around what seems to
be some bugs in doxygen (issues 8635 and 8638).

The @addtogroup command doesn't work for entities inside a nested
namespace (see 8635) so we need to close and reopen groups on entering
and elaving nested namespaces. This fixes the problem that
chrono::duration and chrono::time_point were not documented in the
"Time" documentation group. I am unable to make the path classes appear
as part of their relevant groups (File System and Filesystem TS), nor
the contents of <exception> or <system_error>. I have made some minor
improvements to the docs for those types, including starting to address
PR 97001 by adding @since to the doxygen comments.

This change also excludes the <experimental/bits/net.h> header from
Doxygen processing, so we don't get an unwanted "Networking-ts" group
in the documentation.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	* doc/doxygen/doxygroups.cc: Fix docs for std::literals.
	* doc/doxygen/user.cfg.in: Exclude the Networking TS header.
	Add some more predefined macros.
	* include/bits/fs_fwd.h: Move @addtogroup commands inside
	namespaces. Add better documentation.
	* include/bits/fs_path.h: Likewise.
	* include/experimental/bits/fs_fwd.h: Likewise.
	* include/experimental/bits/fs_path.h: Likewise.
	* include/ext/throw_allocator.h: Fix typo and improve docs.
	* include/std/chrono: Move @addtogroup commands.
	* include/std/system_error: Move @addtogroup commands.
	* libsupc++/exception: Improve documentation.
	* libsupc++/exception.h: Add @since documentation.