[SPARK-30941][PYSPARK] Add a note to asDict to document its behavior …

…when there are duplicate fields

### What changes were proposed in this pull request?

Adding a note to document `Row.asDict` behavior when there are duplicate fields.

### Why are the changes needed?

When a row contains duplicate fields, `asDict` and `_get_item_` behaves differently. We should document it to let users know the difference explicitly.

### Does this PR introduce any user-facing change?

No. Only document change.

### How was this patch tested?

Existing test.

Closes apache#27853 from viirya/SPARK-30941.

Authored-by: Liang-Chi Hsieh <viirya@gmail.com>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>
(cherry picked from commit d21aab4)
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

python/pyspark/sql/types.py

@@ -1528,6 +1528,12 @@ def asDict(self, recursive=False):
 
         :param recursive: turns the nested Rows to dict (default: False).
 
+        .. note:: If a row contains duplicate field names, e.g., the rows of a join
+            between two :class:`DataFrame` that both have the fields of same names,
+            one of the duplicate fields will be selected by ``asDict``. ``__getitem__``
+            will also return one of the duplicate fields, however returned value might
+            be different to ``asDict``.
+
         .. note:: If a row contains duplicate field names, e.g., the rows of a join
             between two :class:`DataFrame` that both have the fields of same names,
             one of the duplicate fields will be selected by ``asDict``. ``__getitem__``