[SPARK-20403][SQL] Modify the instructions of some functions

## What changes were proposed in this pull request?
1.    add  instructions of  'cast'  function When using 'show functions'  and 'desc function cast'
       command in spark-sql
2.    Modify the  instructions of functions，such as
     boolean，tinyint，smallint，int，bigint，float，double，decimal，date，timestamp，binary，string

## How was this patch tested?
Before modification：
spark-sql>desc function boolean;
Function: boolean
Class: org.apache.spark.sql.catalyst.expressions.Cast
Usage: boolean(expr AS type) - Casts the value `expr` to the target data type `type`.

After modification：
spark-sql> desc function boolean;
Function: boolean
Class: org.apache.spark.sql.catalyst.expressions.Cast
Usage: boolean(expr) - Casts the value `expr` to the target data type `boolean`.

spark-sql> desc function cast
Function: cast
Class: org.apache.spark.sql.catalyst.expressions.Cast
Usage: cast(expr AS type) - Casts the value `expr` to the target data type `type`.

Author: liuxian <liu.xian3@zte.com.cn>

Closes #17698 from 10110346/wip_lx_0418.