[Ada] Add GNAT specific pragmas to the equivalent Assertion_Policy for -gnata

[Ada] Add GNAT specific pragmas to the equivalent Assertion_Policy for -gnata

All assertion pragmas are enabled by default when using -gnata. We need
to add the GNAT specific ones to the list.

gcc/ada/

	* doc/gnat_ugn/building_executable_programs_with_gnat.rst
	(Debugging and Assertion Control): Add GNAT specific assertion
	pragmas to the equivalent Assertion_Policy for the -gnata
	option.
	* gnat_ugn.texi: Regenerate.