### What changes were proposed in this pull request?
More precise description of the result of the `percentile_approx()` function and its synonym `approx_percentile()`. The proposed sentence clarifies that the function returns **one of elements** (or array of elements) from the input column.
### Why are the changes needed?
To improve Spark docs and avoid misunderstanding of the function behavior.
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
`./dev/scalastyle`
Closes#29835 from MaxGekk/doc-percentile_approx.
Authored-by: Max Gekk <max.gekk@gmail.com>
Signed-off-by: Liang-Chi Hsieh <viirya@gmail.com>
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