[Ada] Remove some ??? comments

[Ada] Remove some ??? comments

gcc/ada/

	* errout.ads (Size_Too_Small_Message): Remove low-value ???
	comment.
	* exp_util.ads: Remove ??? in part of overall package comments
	and restructure comment to clarify.
	(Duplicate_Subexpr): Remove ??? comment that seems unnecessary.
	* sem_ch3.ads (Analyze_Declarations): Remove two parenthesized
	??? comments and add more description of the procedure's
	actions.
	(Get_Discriminant_Value): Remove ??? comment requesting more
	documentation, expanding description of the function's actions.
	* sem_disp.ads (Check_Operation_From_Incomplete_Type): Add more
	semantic description of the procedure and remove ??? comment
	requesting such.
	(Propagate_Tag): Refine comment to indicate meaning of formal
	parameters and generally improve the spec comment (and remove
	??? comment asking about the parameters).