[SPARK-21589][SQL][DOC] Add documents about Hive UDF/UDTF/UDAF

[maropu]

What changes were proposed in this pull request?

This pr added documents about unsupported functions in Hive UDF/UDTF/UDAF.
This pr relates to #18768 and #18527.

How was this patch tested?

N/A

[maropu]

@gatorsmile If you get time, could you check this? Thanks!

[gatorsmile]

to to -> to

[gatorsmile]

functions with MapredContext, which is inapplicable to Spark.

[gatorsmile]

finished -> finish

[gatorsmile]

Please remove Spark SQL does not support this optimization yet

[SparkQA]

Test build #80103 has finished for PR 18792 at commit 1434bde.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[gatorsmile]

How about simplifying the whole paragraph to?

Not all the APIs of the Hive UDF/UDTF/UDAF are supported by Spark SQL. Below are the unsupported APIs:

[gatorsmile]

Thanks for working on it! Just left some minor comments.

[maropu]

@gatorsmile ok, fixed.

[SparkQA]

Test build #80106 has finished for PR 18792 at commit 7d07e6b.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #80107 has finished for PR 18792 at commit c703d57.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[viirya]

nit: But looks redundant here, because there's inapplicable before. Looks like negative to negative...

[maropu]

removed. Thanks!

[gatorsmile]

LGTM pending Jenkins

[SparkQA]

Test build #80109 has finished for PR 18792 at commit 29f1108.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.