PR69699 document why __GLIBCXX__ macro is useless

	PR libstdc++/69699
	* doc/xml/manual/abi.xml (abi.versioning.history): Explain why the
	_GLIBCXX__ macro is not useful. Remove redundant date information
	and link to the GCC release timeline.
	(abi.versioning.active): Move partial sentence into the previous
	paragraph.
	* doc/html/*: Regenerate.

From-SVN: r244532