PR libstdc++/90686
	* doc/xml/manual/status_cxx2014.xml: Document what's missing from
	<experimental/memory_resource>.
	* doc/xml/manual/status_cxx2020.xml: Document status of P1285R0,
	P0339R6, P0340R3, P1164R1 and P1357R1.
	* doc/html/*: Regenerate.

From-SVN: r271867