ada: Fix documentation of Generalized Finalization extension

The current documentation does not reflect the implementation present in
the compiler and contains various other inaccuracies.

gcc/ada/ChangeLog:

	* doc/gnat_rm/gnat_language_extensions.rst
	(Generalized Finalization): Document the actual implementation.
	(No_Raise): Move to separate section.
	* gnat_rm.texi: Regenerate.