libstdc++: Document that _GLIBCXX_CONCEPT_CHECKS might be removed in future

libstdc++: Document that _GLIBCXX_CONCEPT_CHECKS might be removed in future

The macro-based concept checks are unmaintained and do not support C++11
or later, so reject valid code. If nobody plans to update them we should
consider removing them. Alternatively, we could ignore the macro for
C++11 and later, so they have no effect and don't reject valid code.

libstdc++-v3/ChangeLog:

	* doc/xml/manual/debug.xml: Document that concept checking might
	be removed in future.
	* doc/xml/manual/extensions.xml: Likewise.