[Ada] Document pragma Ada_2022

[Ada] Document pragma Ada_2022

Pragma Ada_2022 is similar to existing pragma Ada_2012 and similarly
deserves to be documented.

gcc/ada/

	* doc/gnat_rm/implementation_defined_pragmas.rst (Pragma
	Ada_2022): Copy description from pragma Ada 2012 and adapt.
	* doc/gnat_ugn/the_gnat_compilation_model.rst (Configuration
	Pragmas): List pragma Ada 2022.
	* gnat_rm.texi, gnat_ugn.texi: Regenerate.