### What changes were proposed in this pull request?
\_FUNC\_ is used in note() of `ExpressionDescription` since https://github.com/apache/spark/pull/28248, it can be more cases later, we should replace it with function name for documentation
### Why are the changes needed?
doc fix
### Does this PR introduce any user-facing change?
no
### How was this patch tested?
pass Jenkins, and verify locally with Jekyll serve
Closes#28305 from yaooqinn/SPARK-31474-F.
Authored-by: Kent Yao <yaooqinn@hotmail.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>