Other small code improvements

- is/is not null applied
- some "result is never used" warnings eliminated
- summaries to public extension methods added
- other things

Author: alex-kulakov

Orm/Xtensive.Orm/Modelling/Comparison/Comparer.cs

@@ -490,7 +490,7 @@ from diff in newDiffs
         let nodeDiff = diff as NodeDifference
         where nodeDiff!=null && (nodeDiff.MovementInfo & MovementInfo.Changed)!=0
         select nodeDiff;
-      // query = query.ToList();
+
       return query.Any();
     }