Improve libstdc++ docs w.r.t newer C++ standards

Improve libstdc++ docs w.r.t newer C++ standards

Instead of repeating all the old headers for every new standard I've
changed the docs to only list the new headers for each standard.

	* doc/xml/manual/test.xml: Improve documentation on writing tests for
	newer standards.
	* doc/xml/manual/using.xml: Document all headers for C++11 and later.
	* doc/html/*: Regenerate.

From-SVN: r263163