Add some comments to some SQL features

Otherwise, it could be confusing to a reader that some of these
well-publicized features are simply listed as unsupported without
further explanation.

Author: petere

src/backend/catalog/sql_features.txt

@@ -432,8 +432,8 @@ T171	LIKE clause in table definition			YES
 T172	AS subquery clause in table definition			YES	
 T173	Extended LIKE clause in table definition			YES	
 T174	Identity columns			YES	
-T175	Generated columns			NO	
-T176	Sequence generator support			NO	
+T175	Generated columns			NO	mostly supported
+T176	Sequence generator support			NO	supported except for NEXT VALUE FOR
 T177	Sequence generator support: simple restart option			YES	
 T178	Identity columns:  simple restart option			YES	
 T180	System-versioned tables			NO