ada: Formal package comment corrections in sinfo.ads

ada: Formal package comment corrections in sinfo.ads

Misc comment corrections and clarifications in sinfo.ads
related to generic formal packages.

gcc/ada/

	* sinfo.ads: Misc comment corrections and clarifications.

	The syntax for GENERIC_ASSOCIATION and FORMAL_PACKAGE_ACTUAL_PART
	was wrong.

	Emphasize that "others => <>" is not represented as an
	N_Generic_Association (with or without Box_Present set),
	and give examples illustrating the various possibilities.