libstdc++: Update C++20 library status docs

This reorganises the C++20 status table, grouping the proposals by
category. It also adds more proposals, and documents all the feature
test macros for C++20 library changes.

	* doc/xml/manual/status_cxx2020.xml: Update C++20 status table.
	* doc/html/*: Regenerate.

libstdc++-v3/ChangeLog

 2020-04-23  Jonathan Wakely  <jwakely@redhat.com>
 
+	* doc/xml/manual/status_cxx2020.xml: Update C++20 status table.
+	* doc/html/*: Regenerate.
+
 	* include/bits/stl_iterator.h (__cpp_lib_array_constexpr): Revert
 	value for C++17 to 201803L because P0858R0 is supported for C++17.
 	* include/std/version (__cpp_lib_array_constexpr): Likewise.