[SPARK-18411] [SQL] Add Argument Types and Test Cases for String Functions [WIP]

[gatorsmile]

What changes were proposed in this pull request?

Add argument types and test cases into the extended descriptions of string functions.

How was this patch tested?

Added test cases to verify the added argument types.

[SparkQA]

Test build #68511 has finished for PR 15850 at commit c02b10d.

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

[SparkQA]

Test build #68581 has finished for PR 15850 at commit 1ba3d67.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[HyukjinKwon]

Hi @gatorsmile, is this still WIP?