ada: Fix comments about Subprogram_Variant and Exceptional_Cases

The comment about Subprogram_Variant was outdated after more types have been
allowed by the corresponding SPARK RM rule; the comment about Exceptional_Cases
was incorrect, after being copy-pasted.

gcc/ada/ChangeLog:

	* sem_prag.adb (Analyze_Exceptional_Contract, Analyze_Variant): Fix
	comments.

gcc/ada/sem_prag.adb

@@ -2320,8 +2320,7 @@ package body Sem_Prag is
          Errors := Serious_Errors_Detected;
-         --  Preanalyze_Assert_Expression, but without enforcing any of the two
-         --  acceptable types.
+         --  Preanalyze_Assert_Expression enforcing the expression type
          Preanalyze_Assert_Expression (Consequence, Any_Boolean);
@@ -31380,7 +31379,7 @@ package body Sem_Prag is
          Errors := Serious_Errors_Detected;
-         --  Preanalyze_Assert_Expression, but without enforcing any of the two
+         --  Preanalyze_Assert_Expression, but without enforcing any of the
          --  acceptable types.
          Preanalyze_Assert_Expression (Expr);