[SPARK-9141] [SQL] [MINOR] Fix comments of PR #7920

This is a follow-up of https://github.com/apache/spark/pull/7920 to fix comments.

Author: Yin Huai <yhuai@databricks.com>

Closes #7964 from yhuai/SPARK-9141-follow-up and squashes the following commits:

4d0ee80 [Yin Huai] Fix comments.

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

@@ -140,7 +140,7 @@ class Analyzer(
   object ResolveAliases extends Rule[LogicalPlan] {
     private def assignAliases(exprs: Seq[NamedExpression]) = {
       // The `UnresolvedAlias`s will appear only at root of a expression tree, we don't need
-      // to resolveOperator down the whole tree.
+      // to traverse the whole tree.
       exprs.zipWithIndex.map {
         case (u @ UnresolvedAlias(child), i) =>
           child match {
@@ -873,7 +873,6 @@ class Analyzer(
     // "Aggregate with Having clause" will be triggered.
     def apply(plan: LogicalPlan): LogicalPlan = plan transformDown {
 
-
       // Aggregate with Having clause. This rule works with an unresolved Aggregate because
       // a resolved Aggregate will not have Window Functions.
       case f @ Filter(condition, a @ Aggregate(groupingExprs, aggregateExprs, child))

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/plans/LogicalPlanSuite.scala

@@ -23,7 +23,8 @@ import org.apache.spark.sql.catalyst.plans.logical._
 import org.apache.spark.sql.catalyst.util._
 
 /**
- * Provides helper methods for comparing plans.
+ * This suite is used to test [[LogicalPlan]]'s `resolveOperators` and make sure it can correctly
+ * skips sub-trees that have already been marked as analyzed.
  */
 class LogicalPlanSuite extends SparkFunSuite {
   private var invocationCount = 0