ada: Pragma Ada_XX not propagated from library level spec to body

ada: Pragma Ada_XX not propagated from library level spec to body

Add documentation to pragmas Ada_83, Ada_95, Ada_05, Ada_12,
and Ada_2022: when placed before a library level package
specification they are not propagated to the corresponding
package body; they must be added explicitly to the package
body.

gcc/ada/ChangeLog:

	* doc/gnat_rm/implementation_defined_pragmas.rst: Adding
	documentation.
	* doc/gnat_ugn/the_gnat_compilation_model.rst: ditto.
	* gnat_rm.texi: Regenerate.
	* gnat_ugn.texi: Regenerate.