[SPARK-23861][SQL][DOC] Clarify default window frame with and without orderBy clause

## What changes were proposed in this pull request?

Add docstring to clarify default window frame boundaries with and without orderBy clause

## How was this patch tested?

Manually generate doc and check.

Author: Li Jin <[email protected]>

Closes apache#20978 from icexelloss/SPARK-23861-window-doc.

Author: icexelloss

python/pyspark/sql/window.py

@@ -44,6 +44,10 @@ class Window(object):
     >>> # PARTITION BY country ORDER BY date RANGE BETWEEN 3 PRECEDING AND 3 FOLLOWING
     >>> window = Window.orderBy("date").partitionBy("country").rangeBetween(-3, 3)
 
+    .. note:: When ordering is not defined, an unbounded window frame (rowFrame,
+         unboundedPreceding, unboundedFollowing) is used by default. When ordering is defined,
+         a growing window frame (rangeFrame, unboundedPreceding, currentRow) is used by default.
+
     .. note:: Experimental
 
     .. versionadded:: 1.4

sql/core/src/main/scala/org/apache/spark/sql/expressions/Window.scala

@@ -33,6 +33,10 @@ import org.apache.spark.sql.catalyst.expressions._
  *   Window.partitionBy("country").orderBy("date").rowsBetween(-3, 3)
  * }}}
  *
+ * @note When ordering is not defined, an unbounded window frame (rowFrame, unboundedPreceding,
+ *       unboundedFollowing) is used by default. When ordering is defined, a growing window frame
+ *       (rangeFrame, unboundedPreceding, currentRow) is used by default.
+ *
  * @since 1.4.0
  */
 @InterfaceStability.Stable