[Ada] Document attribute Has_Tagged_Values in GNAT RM

[Ada] Document attribute Has_Tagged_Values in GNAT RM

gcc/ada/

	* doc/gnat_rm/implementation_defined_attributes.rst
	(Has_Tagged_Values): Document based on the existing description
	of Has_Access_Type and the comment for Has_Tagged_Component,
	which is where frontend evaluates this attribute.
	* gnat_rm.texi: Regenerate.
	* sem_attr.adb (Analyze_Attribute): Merge processing of
	Has_Access_Type and Has_Tagged_Component attributes.
	* sem_util.adb (Has_Access_Type): Fix casing in comment.
	* sem_util.ads (Has_Tagged_Component): Remove wrong (or
	outdated) comment about the use of this routine to implement the
	equality operator.