c++: improve comment

c++: improve comment

It's incorrect to say that the address of an OFFSET_REF is always a
pointer-to-member; if it represents an overload set with both static and
non-static member functions that ends up resolving to a static one, the
address is a normal pointer.  And let's go ahead and mention explicit object
member functions even though the patch hasn't landed yet.

gcc/cp/ChangeLog:

	* cp-tree.def: Improve OFFSET_REF comment.
	* cp-gimplify.cc (cp_fold_immediate): Add to comment.