[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.

(cherry picked from commit 197f901)
Signed-off-by: Xiao Li <gatorsmile@gmail.com>

Author: 10110346

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/FunctionRegistry.scala

@@ -430,6 +430,8 @@ object FunctionRegistry {
     expression[StructsToJson]("to_json"),
     expression[JsonToStructs]("from_json"),
 
+    // cast
+    expression[Cast]("cast"),
     // Cast aliases (SPARK-16730)
     castAlias("boolean", BooleanType),
     castAlias("tinyint", ByteType),
@@ -512,7 +514,9 @@ object FunctionRegistry {
       }
       Cast(args.head, dataType)
     }
-    (name, (expressionInfo[Cast](name), builder))
+    val clazz = scala.reflect.classTag[Cast].runtimeClass
+    val usage = "_FUNC_(expr) - Casts the value `expr` to the target data type `_FUNC_`."
+    (name, (new ExpressionInfo(clazz.getCanonicalName, null, name, usage, null), builder))
   }
 
   /**

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/mathExpressions.scala

@@ -966,7 +966,7 @@ case class Logarithm(left: Expression, right: Expression)
  *
  * @param child expr to be round, all [[NumericType]] is allowed as Input
  * @param scale new scale to be round to, this should be a constant int at runtime
- * @param mode rounding mode (e.g. HALF_UP, HALF_UP)
+ * @param mode rounding mode (e.g. HALF_UP, HALF_EVEN)
  * @param modeStr rounding mode string name (e.g. "ROUND_HALF_UP", "ROUND_HALF_EVEN")
  */
 abstract class RoundBase(child: Expression, scale: Expression,

sql/core/src/test/resources/sql-tests/inputs/cast.sql

@@ -40,4 +40,6 @@ SELECT CAST('-9223372036854775809' AS long);
 SELECT CAST('9223372036854775807' AS long);
 SELECT CAST('9223372036854775808' AS long);
 
+DESC FUNCTION boolean;
+DESC FUNCTION EXTENDED boolean;
 -- TODO: migrate all cast tests here.

sql/core/src/test/resources/sql-tests/results/cast.sql.out

@@ -1,5 +1,5 @@
 -- Automatically generated by SQLQueryTestSuite
--- Number of queries: 22
+-- Number of queries: 24
 
 
 -- !query 0
@@ -176,3 +176,24 @@ SELECT CAST('9223372036854775808' AS long)
 struct<CAST(9223372036854775808 AS BIGINT):bigint>
 -- !query 21 output
 NULL
+
+
+-- !query 22
+DESC FUNCTION boolean
+-- !query 22 schema
+struct<function_desc:string>
+-- !query 22 output
+Class: org.apache.spark.sql.catalyst.expressions.Cast
+Function: boolean
+Usage: boolean(expr) - Casts the value `expr` to the target data type `boolean`.
+
+
+-- !query 23
+DESC FUNCTION EXTENDED boolean
+-- !query 23 schema
+struct<function_desc:string>
+-- !query 23 output
+Class: org.apache.spark.sql.catalyst.expressions.Cast
+Extended Usage:N/A.
+Function: boolean
+Usage: boolean(expr) - Casts the value `expr` to the target data type `boolean`.