libstdc++: Document removal of implicit allocator rebinding extensions

libstdc++: Document removal of implicit allocator rebinding extensions

Traditionally libstdc++ allowed containers to be
instantiated with allocator's that have the wrong value type, implicitly
rebinding the allocator to the container's value type. Since C++20 that
has been explicitly ill-formed, so the extension is no longer supported
in strict modes (e.g. -std=c++17) and in C++20 and later.

libstdc++-v3/ChangeLog:

	* doc/xml/manual/evolution.xml: Document removal of implicit
	allocator rebinding extensions in strict mode and for C++20.
	* doc/html/*: Regenerate.

(cherry picked from commit 8cbaf679a3c1875c5475bd1cb0fb86fb9d03b2d4)