[Ada] Document change to legality checks for Inox case statements

[Ada] Document change to legality checks for Inox case statements

INOX (which is enabled via -gnatX) supports composite case-statement selectors.
As a temporary measure, simplify the coverage-related compile-time checks
for such case statements via two changes: an others choice is always
required for such a case statement, and no legality checks relating to
overlapping of case choices are performed.

gcc/ada/

	* doc/gnat_rm/implementation_defined_pragmas.rst: Document new
	temporary rule that a "when others =>" case choice must be given
	when casing on a composite selector.
	* gnat_rm.texi: Regenerate.