[SPARK-12456][SQL] Add ExpressionDescription to misc functions

First try, not sure how much information we need to provide in the usage part.

Author: Xiu Guo <xguo27@gmail.com>

Closes #10423 from xguo27/SPARK-12456.

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

@@ -26,6 +26,9 @@ import org.apache.spark.unsafe.types.UTF8String
 /**
  * Expression that returns the name of the current file being read in using [[SqlNewHadoopRDD]]
  */
+@ExpressionDescription(
+  usage = "_FUNC_() - Returns the name of the current file being read if available",
+  extended = "> SELECT _FUNC_();\n ''")
 case class InputFileName() extends LeafExpression with Nondeterministic {
 
   override def nullable: Boolean = true

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

@@ -32,6 +32,14 @@ import org.apache.spark.sql.types.{LongType, DataType}
  *
  * Since this expression is stateful, it cannot be a case object.
  */
+@ExpressionDescription(
+  usage =
+    """_FUNC_() - Returns monotonically increasing 64-bit integers.
+      The generated ID is guaranteed to be monotonically increasing and unique, but not consecutive.
+      The current implementation puts the partition ID in the upper 31 bits, and the lower 33 bits
+      represent the record number within each partition. The assumption is that the data frame has
+      less than 1 billion partitions, and each partition has less than 8 billion records.""",
+  extended = "> SELECT _FUNC_();\n 0")
 private[sql] case class MonotonicallyIncreasingID() extends LeafExpression with Nondeterministic {
 
   /**

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

@@ -26,6 +26,9 @@ import org.apache.spark.sql.types.{IntegerType, DataType}
 /**
  * Expression that returns the current partition id of the Spark task.
  */
+@ExpressionDescription(
+  usage = "_FUNC_() - Returns the current partition id of the Spark task",
+  extended = "> SELECT _FUNC_();\n 0")
 private[sql] case class SparkPartitionID() extends LeafExpression with Nondeterministic {
 
   override def nullable: Boolean = false

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

@@ -30,6 +30,9 @@ import org.apache.spark.unsafe.types.UTF8String
  * A function that calculates an MD5 128-bit checksum and returns it as a hex string
  * For input of type [[BinaryType]]
  */
+@ExpressionDescription(
+  usage = "_FUNC_(input) - Returns an MD5 128-bit checksum as a hex string of the input",
+  extended = "> SELECT _FUNC_('Spark');\n '8cde774d6f7333752ed72cacddb05126'")
 case class Md5(child: Expression) extends UnaryExpression with ImplicitCastInputTypes {
 
   override def dataType: DataType = StringType
@@ -53,6 +56,12 @@ case class Md5(child: Expression) extends UnaryExpression with ImplicitCastInput
  * asking for an unsupported SHA function, the return value is NULL. If either argument is NULL or
  * the hash length is not one of the permitted values, the return value is NULL.
  */
+@ExpressionDescription(
+  usage = "_FUNC_(input, bitLength) - Returns a checksum of SHA-2 family as a hex string of the " +
+    "input. SHA-224, SHA-256, SHA-384, and SHA-512 are supported. Bit length of 0 is equivalent " +
+    "to 256",
+  extended = "> SELECT _FUNC_('Spark', 0);\n " +
+    "'529bc3b07127ecb7e53a4dcf1991d9152c24537d919178022b2c42657f79a26b'")
 case class Sha2(left: Expression, right: Expression)
   extends BinaryExpression with Serializable with ImplicitCastInputTypes {
 
@@ -118,6 +127,9 @@ case class Sha2(left: Expression, right: Expression)
  * A function that calculates a sha1 hash value and returns it as a hex string
  * For input of type [[BinaryType]] or [[StringType]]
  */
+@ExpressionDescription(
+  usage = "_FUNC_(input) - Returns a sha1 hash value as a hex string of the input",
+  extended = "> SELECT _FUNC_('Spark');\n '85f5955f4b27a9a4c2aab6ffe5d7189fc298b92c'")
 case class Sha1(child: Expression) extends UnaryExpression with ImplicitCastInputTypes {
 
   override def dataType: DataType = StringType
@@ -138,6 +150,9 @@ case class Sha1(child: Expression) extends UnaryExpression with ImplicitCastInpu
  * A function that computes a cyclic redundancy check value and returns it as a bigint
  * For input of type [[BinaryType]]
  */
+@ExpressionDescription(
+  usage = "_FUNC_(input) - Returns a cyclic redundancy check value as a bigint of the input",
+  extended = "> SELECT _FUNC_('Spark');\n '1557323817'")
 case class Crc32(child: Expression) extends UnaryExpression with ImplicitCastInputTypes {
 
   override def dataType: DataType = LongType