[SPARK-31331][SQL][DOCS] Document Spark integration with Hive UDFs/UDAFs/UDTFs
[SPARK-31331][SQL][DOCS] Document Spark integration with Hive UDFs/UDAFs/UDTFs

### What changes were proposed in this pull request?
Document Spark integration with Hive UDFs/UDAFs/UDTFs

### Why are the changes needed?
To make SQL Reference complete

### Does this PR introduce any user-facing change?
Yes
<img width="1031" alt="Screen Shot 2020-04-02 at 2 22 42 PM" src="https://user-images.githubusercontent.com/13592258/78301971-cc7cf080-74ee-11ea-93c8-7d4c75213b47.png">

### How was this patch tested?
Manually build and check

Closes #28104 from huaxingao/hive-udfs.

Lead-authored-by: Huaxin Gao <b1011540267ee7fe5a02243ad09c007ca1860809@us.ibm.com>
Co-authored-by: Takeshi Yamamuro <9770f4f4b25a32d3dfa34670a10767defeb974f3@apache.org>
Signed-off-by: Sean Owen <d519fa33a62684a61ab04b24d26f83452d51b34c@gmail.com>

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