197f9018a4
## 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.
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