ada: Generic formal/actual matching -- misc cleanup

ada: Generic formal/actual matching -- misc cleanup

The only substantive change is to remove Activation_Chain_Entity
from N_Generic_Package_Declaration. The comment in sinfo.ads suggesting
this change was written in 1993!

Various pieces of missing documentation are added to Sinfo and Einfo.

Also other minor cleanups.

gcc/ada/

	* gen_il-gen-gen_nodes.adb
	(N_Generic_Package_Declaration): Remove Activation_Chain_Entity.
	* sinfo.ads: Comment improvements. Add missing doc.
	Remove obsolete comment about Activation_Chain_Entity.
	* einfo.ads: Comment improvements. Add missing doc.
	* einfo-utils.adb (Base_Type): Add Assert (disabled for now).
	(Next_Index): Minor cleanup.
	* aspects.ads: Minor comment fix.
	* exp_ch6.adb: Likewise.
	* sem_ch3.adb: Likewise.