* doc/xml/manual/abi.xml: Add xml:id anchor.
	* doc/xml/manual/using.xml (manual.intro.using.macros): Document
	_GLIBCXX_RELEASE. Link to new anchor for __GLIBCXX__ notes.
	(concurrency.io.structure): Add markup.
	* doc/html/*: Regenerate.

From-SVN: r246532