### 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#27853 from viirya/SPARK-30941.
Authored-by: Liang-Chi Hsieh <viirya@gmail.com>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>
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