[SQL] some comments fix for GROUPING SETS

Author: Daoyuan Wang <daoyuan.wang@intel.com>

Closes #4000 from adrian-wang/comment and squashes the following commits:

9c24fc4 [Daoyuan Wang] some comments

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/Analyzer.scala

@@ -126,10 +126,10 @@ class Analyzer(catalog: Catalog,
     }
 
     /*
-     *  GROUP BY a, b, c, WITH ROLLUP
+     *  GROUP BY a, b, c WITH ROLLUP
      *  is equivalent to
-     *  GROUP BY a, b, c GROUPING SETS ( (a, b, c), (a, b), (a), ( )).
-     *  Group Count: N + 1 (N is the number of group expression)
+     *  GROUP BY a, b, c GROUPING SETS ( (a, b, c), (a, b), (a), ( ) ).
+     *  Group Count: N + 1 (N is the number of group expressions)
      *
      *  We need to get all of its subsets for the rule described above, the subset is
      *  represented as the bit masks.
@@ -139,12 +139,12 @@ class Analyzer(catalog: Catalog,
     }
 
     /*
-     *  GROUP BY a, b, c, WITH CUBE
+     *  GROUP BY a, b, c WITH CUBE
      *  is equivalent to
      *  GROUP BY a, b, c GROUPING SETS ( (a, b, c), (a, b), (b, c), (a, c), (a), (b), (c), ( ) ).
-     *  Group Count: 2^N (N is the number of group expression)
+     *  Group Count: 2 ^ N (N is the number of group expressions)
      *
-     *  We need to get all of its sub sets for a given GROUPBY expressions, the subset is
+     *  We need to get all of its subsets for a given GROUPBY expression, the subsets are
      *  represented as the bit masks.
      */
     def bitmasks(c: Cube): Seq[Int] = {