[Ada] Sync documentation of junk names with code

[Ada] Sync documentation of junk names with code

The list of junk entity names for which warnings are suppressed has been
recently modified, so update the documentation accordingly.

gcc/ada/

	* doc/gnat_rm/implementation_defined_pragmas.rst
	(Pragma Unreferenced): Sync description with
	Sem_Warn.Has_Junk_Name routine.
	* gnat_rm.texi: Regenerate.
	* gnat_ugn.texi: Regenerate.