[SPARK-38391][SPARK-38768][SQL][FOLLOWUP] Add comments for `pushLimit…

…` and `pushTopN` of `PushDownUtils`

### What changes were proposed in this pull request?
`pushLimit` and `pushTopN` of `PushDownUtils` returns tuple of boolean. It will be good to explain what the boolean value represents.

### Why are the changes needed?
Make DS V2 API more friendly to developers.

### Does this PR introduce _any_ user-facing change?
'No'.
Just update comments.

### How was this patch tested?
N/A

Closes #36092 from beliefer/SPARK-38391_SPARK-38768_followup.

Authored-by: Jiaan Geng <beliefer@163.com>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/v2/PushDownUtils.scala

@@ -116,7 +116,11 @@ object PushDownUtils extends PredicateHelper {
   }
 
   /**
-   * Pushes down LIMIT to the data source Scan
+   * Pushes down LIMIT to the data source Scan.
+   *
+   * @return the tuple of Boolean. The first Boolean value represents whether to push down, and
+   *         the second Boolean value represents whether to push down partially, which means
+   *         Spark will keep the Limit and do it again.
    */
   def pushLimit(scanBuilder: ScanBuilder, limit: Int): (Boolean, Boolean) = {
     scanBuilder match {
@@ -127,7 +131,11 @@ object PushDownUtils extends PredicateHelper {
   }
 
   /**
-   * Pushes down top N to the data source Scan
+   * Pushes down top N to the data source Scan.
+   *
+   * @return the tuple of Boolean. The first Boolean value represents whether to push down, and
+   *         the second Boolean value represents whether to push down partially, which means
+   *         Spark will keep the Sort and Limit and do it again.
    */
   def pushTopN(
       scanBuilder: ScanBuilder,