ada: Clear confusion about Standard'Maximum_Alignment

ada: Clear confusion about Standard'Maximum_Alignment

This documents the meaning of Standard'Maximum_Alignment as it has been
implemented in the GCC-based compiler for more than a decade.

gcc/ada/ChangeLog:

	* doc/gnat_rm/implementation_defined_attributes.rst
	(Maximum_Alignment): Fix description.
	* doc/gnat_rm/representation_clauses_and_pragmas.rst
	(Alignment Clauses): Adjust accordingly.
	* checks.adb (Apply_Address_Clause_Check): Remove incorrect test on
	Maximum_Alignment.
	* sem_ch13.adb (Analyze_Attribute_Definition_Clause): Minor tweak
	in comment.
	* ttypes.ads (Maximum_Alignment): Fix description.
	* gnat_rm.texi: Regenerate.