extend.texi (Function Attributes): Rewrite visibility attribute documentation.

	* doc/extend.texi (Function Attributes): Rewrite visibility 
	attribute documentation.
	* doc/invoke.texi (C++ Dialect Options): Rewrite
	-fvisibility-inlines-hidden documentation to describe something
	entirely different, although in practise compatible.
	(Code Gen Options): Warn about system headers in -fvisibiltity=
	documentation.

From-SVN: r112653