Update comments.

Author: yhuai

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala

@@ -532,7 +532,8 @@ object CombineFilters extends Rule[LogicalPlan] {
     case Filter(Not(AtLeastNNulls(1, e1)), Filter(Not(AtLeastNNulls(1, e2)), grandChild)) =>
       // If we are combining two expressions Not(AtLeastNNulls(1, e1)) and
       // Not(AtLeastNNulls(1, e2))
-      // (this is used to make sure there is no null in the result of e1 and e2), we can
+      // (this is used to make sure there is no null in the result of e1 and e2 and
+      // they are added by FilterNullsInJoinKey optimziation rule), we can
       // just create a Not(AtLeastNNulls(1, (e1 ++ e2).distinct)).
       Filter(Not(AtLeastNNulls(1, (e1 ++ e2).distinct)), grandChild)
     case ff @ Filter(fc, nf @ Filter(nc, grandChild)) => Filter(And(nc, fc), grandChild)

sql/core/src/test/scala/org/apache/spark/sql/optimizer/FilterNullsInJoinKeySuite.scala

@@ -27,7 +27,7 @@ import org.apache.spark.sql.catalyst.plans.logical.{Filter, LocalRelation, Logic
 import org.apache.spark.sql.catalyst.rules.RuleExecutor
 import org.apache.spark.sql.test.TestSQLContext
 
-/** This is the test suite for FilterNullsInJoinKey optimziation rule. */
+/** This is the test suite for FilterNullsInJoinKey optimization rule. */
 class FilterNullsInJoinKeySuite extends PlanTest {
 
   // We add predicate pushdown rules at here to make sure we do not