[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 <huaxing@us.ibm.com>
Co-authored-by: Takeshi Yamamuro <yamamuro@apache.org>
Signed-off-by: Sean Owen <srowen@gmail.com>

docs/sql-ref-functions-udf-hive.md

@@ -19,4 +19,90 @@ license: |
   limitations under the License.
 ---
 
-Integration with Hive UDFs/UDAFs/UDTFs
+### Description
+
+Spark SQL supports integration of Hive UDFs, UDAFs and UDTFs. Similar to Spark UDFs and UDAFs, Hive UDFs work on a single row as input and generate a single row as output, while Hive UDAFs operate on multiple rows and return a single aggregated row as a result. In addition, Hive also supports UDTFs (User Defined Tabular Functions) that act on one row as input and return multiple rows as output. To use Hive UDFs/UDAFs/UTFs, the user should register them in Spark, and then use them in Spark SQL queries.
+
+### Examples
+
+Hive has two UDF interfaces: [UDF](https://github.com/apache/hive/blob/master/udf/src/java/org/apache/hadoop/hive/ql/exec/UDF.java) and [GenericUDF](https://github.com/apache/hive/blob/master/ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDF.java).
+An example below uses [GenericUDFAbs](https://github.com/apache/hive/blob/master/ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDFAbs.java) derived from `GenericUDF`.
+
+{% highlight sql %}
+-- Register `GenericUDFAbs` and use it in Spark SQL.
+-- Note that, if you use your own programmed one, you need to add a JAR containig it
+-- into a classpath,
+-- e.g., ADD JAR yourHiveUDF.jar;
+CREATE TEMPORARY FUNCTION testUDF AS 'org.apache.hadoop.hive.ql.udf.generic.GenericUDFAbs';
+
+SELECT * FROM t;
+  +-----+
+  |value|
+  +-----+
+  | -1.0|
+  |  2.0|
+  | -3.0|
+  +-----+
+
+SELECT testUDF(value) FROM t;
+  +--------------+
+  |testUDF(value)|
+  +--------------+
+  |           1.0|
+  |           2.0|
+  |           3.0|
+  +--------------+
+{% endhighlight %}
+
+
+An example below uses [GenericUDTFExplode](https://github.com/apache/hive/blob/master/ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDTFExplode.java) derived from [GenericUDTF](https://github.com/apache/hive/blob/master/ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDF.java).
+
+{% highlight sql %}
+-- Register `GenericUDTFExplode` and use it in Spark SQL
+CREATE TEMPORARY FUNCTION hiveUDTF
+    AS 'org.apache.hadoop.hive.ql.udf.generic.GenericUDTFExplode';
+
+SELECT * FROM t;
+  +------+
+  | value|
+  +------+
+  |[1, 2]|
+  |[3, 4]|
+  +------+
+
+SELECT hiveUDTF(value) FROM t;
+  +---+
+  |col|
+  +---+
+  |  1|
+  |  2|
+  |  3|
+  |  4|
+  +---+
+{% endhighlight %}
+
+Hive has two UDAF interfaces: [UDAF](https://github.com/apache/hive/blob/master/udf/src/java/org/apache/hadoop/hive/ql/exec/UDAF.java) and [GenericUDAFResolver](https://github.com/apache/hive/blob/master/ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDAFResolver.java).
+An example below uses [GenericUDAFSum](https://github.com/apache/hive/blob/master/ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDAFSum.java) derived from `GenericUDAFResolver`.
+
+{% highlight sql %}
+-- Register `GenericUDAFSum` and use it in Spark SQL
+CREATE TEMPORARY FUNCTION hiveUDAF
+    AS 'org.apache.hadoop.hive.ql.udf.generic.GenericUDAFSum';
+
+SELECT * FROM t;
+  +---+-----+
+  |key|value|
+  +---+-----+
+  |  a|    1|
+  |  a|    2|
+  |  b|    3|
+  +---+-----+
+
+SELECT key, hiveUDAF(value) FROM t GROUP BY key;
+  +---+---------------+
+  |key|hiveUDAF(value)|
+  +---+---------------+
+  |  b|              3|
+  |  a|              3|
+  +---+---------------+
+{% endhighlight %}