[MINOR][SQL] Improve readability for window execution

### What changes were proposed in this pull request?
I read the comments of `WindowExec` and found some comment will cause confusion and another need to improve.

### Why are the changes needed?
This PR will enhance the readability and let developer works more easy

### Does this PR introduce any user-facing change?
No

### How was this patch tested?
No need

Closes #27431 from beliefer/improve-window-readability.

Authored-by: beliefer <beliefer@163.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

sql/core/src/main/scala/org/apache/spark/sql/execution/window/WindowExec.scala

@@ -38,15 +38,25 @@ import org.apache.spark.sql.types.{CalendarIntervalType, DateType, IntegerType,
  * - Entire partition: The frame is the entire partition, i.e.
  *   UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING. For this case, window function will take all
  *   rows as inputs and be evaluated once.
- * - Growing frame: We only add new rows into the frame, i.e. UNBOUNDED PRECEDING AND ....
+ * - Growing frame: We only add new rows into the frame, Examples are:
+ *     1. UNBOUNDED PRECEDING AND 1 PRECEDING
+ *     2. UNBOUNDED PRECEDING AND CURRENT ROW
+ *     3. UNBOUNDED PRECEDING AND 1 FOLLOWING
  *   Every time we move to a new row to process, we add some rows to the frame. We do not remove
  *   rows from this frame.
- * - Shrinking frame: We only remove rows from the frame, i.e. ... AND UNBOUNDED FOLLOWING.
+ * - Shrinking frame: We only remove rows from the frame, Examples are:
+ *     1. 1 PRECEDING AND UNBOUNDED FOLLOWING
+ *     2. CURRENT ROW AND UNBOUNDED FOLLOWING
+ *     3. 1 FOLLOWING AND UNBOUNDED FOLLOWING
  *   Every time we move to a new row to process, we remove some rows from the frame. We do not add
  *   rows to this frame.
  * - Moving frame: Every time we move to a new row to process, we remove some rows from the frame
  *   and we add some rows to the frame. Examples are:
- *     1 PRECEDING AND CURRENT ROW and 1 FOLLOWING AND 2 FOLLOWING.
+ *     1. 2 PRECEDING AND 1 PRECEDING
+ *     2. 1 PRECEDING AND CURRENT ROW
+ *     3. CURRENT ROW AND 1 FOLLOWING
+ *     4. 1 PRECEDING AND 1 FOLLOWING
+ *     5. 1 FOLLOWING AND 2 FOLLOWING
  * - Offset frame: The frame consist of one row, which is an offset number of rows away from the
  *   current row. Only [[OffsetWindowFunction]]s can be processed in an offset frame.
  *
@@ -105,7 +115,7 @@ case class WindowExec(
   override def outputPartitioning: Partitioning = child.outputPartitioning
 
   protected override def doExecute(): RDD[InternalRow] = {
-    // Unwrap the expressions and factories from the map.
+    // Unwrap the window expressions and window frame factories from the map.
     val expressions = windowFrameExpressionFactoryPairs.flatMap(_._1)
     val factories = windowFrameExpressionFactoryPairs.map(_._2).toArray
     val inMemoryThreshold = sqlContext.conf.windowExecBufferInMemoryThreshold

sql/core/src/main/scala/org/apache/spark/sql/execution/window/WindowExecBase.scala

@@ -114,7 +114,7 @@ abstract class WindowExecBase(
 
   /**
    * Collection containing an entry for each window frame to process. Each entry contains a frame's
-   * [[WindowExpression]]s and factory function for the WindowFrameFunction.
+   * [[WindowExpression]]s and factory function for the [[WindowFrameFunction]].
    */
   protected lazy val windowFrameExpressionFactoryPairs = {
     type FrameKey = (String, FrameType, Expression, Expression)
@@ -170,7 +170,7 @@ abstract class WindowExecBase(
               MutableProjection.create(expressions, schema))
         }
 
-        // Create the factory
+        // Create the factory to produce WindowFunctionFrame.
         val factory = key match {
           // Offset Frame
           case ("OFFSET", _, IntegerLiteral(offset), _) =>
@@ -223,7 +223,7 @@ abstract class WindowExecBase(
         // Keep track of the number of expressions. This is a side-effect in a map...
         numExpressions += expressions.size
 
-        // Create the Frame Expression - Factory pair.
+        // Create the Window Expression - Frame Factory pair.
         (expressions, factory)
     }
   }

sql/core/src/main/scala/org/apache/spark/sql/execution/window/WindowFunctionFrame.scala

@@ -47,14 +47,14 @@ abstract class WindowFunctionFrame {
   /**
    * The current lower window bound in the row array (inclusive).
    *
-   * This should be called after the current row is updated via [[write]]
+   * This should be called after the current row is updated via `write`.
    */
   def currentLowerBound(): Int
 
   /**
    * The current row index of the upper window bound in the row array (exclusive)
    *
-   * This should be called after the current row is updated via [[write]]
+   * This should be called after the current row is updated via `write`.
    */
   def currentUpperBound(): Int
 }