[SPARK-14415][SQL] All functions should show usages by command `DESC FUNCTION`

## What changes were proposed in this pull request?

Currently, many functions do now show usages like the followings.
```
scala> sql("desc function extended `sin`").collect().foreach(println)
[Function: sin]
[Class: org.apache.spark.sql.catalyst.expressions.Sin]
[Usage: To be added.]
[Extended Usage:
To be added.]
```

This PR adds descriptions for functions and adds a testcase prevent adding function without usage.
```
scala>  sql("desc function extended `sin`").collect().foreach(println);
[Function: sin]
[Class: org.apache.spark.sql.catalyst.expressions.Sin]
[Usage: sin(x) - Returns the sine of x.]
[Extended Usage:
> SELECT sin(0);
 0.0]
```

The only exceptions are `cube`, `grouping`, `grouping_id`, `rollup`, `window`.

## How was this patch tested?

Pass the Jenkins tests (including new testcases.)

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #12185 from dongjoon-hyun/SPARK-14415.