ada: Tweak documentation comments

ada: Tweak documentation comments

The concept of extended nodes was retired at the same time Gen_IL
was introduced, but there was a reference to that concept left over
in a comment. This patch removes that reference.

Also, the description of the field Comes_From_Check_Or_Contract was
incorrectly placed in a section for fields present in all nodes in
sinfo.ads. This patch fixes this.

gcc/ada/

	* atree.ads, nlists.ads, types.ads: Remove references to extended
	nodes. Fix typo.
	* sinfo.ads: Likewise and fix position of
	Comes_From_Check_Or_Contract description.