[SPARK-11900][SQL] Add since version for all encoders

Author: Reynold Xin <rxin@databricks.com>

Closes #9881 from rxin/SPARK-11900.

sql/catalyst/src/main/scala/org/apache/spark/sql/Encoder.scala

@@ -45,13 +45,52 @@ trait Encoder[T] extends Serializable {
  */
 object Encoders {
 
+  /**
+   * An encoder for nullable boolean type.
+   * @since 1.6.0
+   */
   def BOOLEAN: Encoder[java.lang.Boolean] = ExpressionEncoder()
+
+  /**
+   * An encoder for nullable byte type.
+   * @since 1.6.0
+   */
   def BYTE: Encoder[java.lang.Byte] = ExpressionEncoder()
+
+  /**
+   * An encoder for nullable short type.
+   * @since 1.6.0
+   */
   def SHORT: Encoder[java.lang.Short] = ExpressionEncoder()
+
+  /**
+   * An encoder for nullable int type.
+   * @since 1.6.0
+   */
   def INT: Encoder[java.lang.Integer] = ExpressionEncoder()
+
+  /**
+   * An encoder for nullable long type.
+   * @since 1.6.0
+   */
   def LONG: Encoder[java.lang.Long] = ExpressionEncoder()
+
+  /**
+   * An encoder for nullable float type.
+   * @since 1.6.0
+   */
   def FLOAT: Encoder[java.lang.Float] = ExpressionEncoder()
+
+  /**
+   * An encoder for nullable double type.
+   * @since 1.6.0
+   */
   def DOUBLE: Encoder[java.lang.Double] = ExpressionEncoder()
+
+  /**
+   * An encoder for nullable string type.
+   * @since 1.6.0
+   */
   def STRING: Encoder[java.lang.String] = ExpressionEncoder()
 
   /**
@@ -59,6 +98,8 @@ object Encoders {
    * This encoder maps T into a single byte array (binary) field.
    *
    * T must be publicly accessible.
+   *
+   * @since 1.6.0
    */
   def kryo[T: ClassTag]: Encoder[T] = genericSerializer(useKryo = true)
 
@@ -67,6 +108,8 @@ object Encoders {
    * This encoder maps T into a single byte array (binary) field.
    *
    * T must be publicly accessible.
+   *
+   * @since 1.6.0
    */
   def kryo[T](clazz: Class[T]): Encoder[T] = kryo(ClassTag[T](clazz))
 
@@ -77,6 +120,8 @@ object Encoders {
    * Note that this is extremely inefficient and should only be used as the last resort.
    *
    * T must be publicly accessible.
+   *
+   * @since 1.6.0
    */
   def javaSerialization[T: ClassTag]: Encoder[T] = genericSerializer(useKryo = false)
 
@@ -87,6 +132,8 @@ object Encoders {
    * Note that this is extremely inefficient and should only be used as the last resort.
    *
    * T must be publicly accessible.
+   *
+   * @since 1.6.0
    */
   def javaSerialization[T](clazz: Class[T]): Encoder[T] = javaSerialization(ClassTag[T](clazz))
 
@@ -120,12 +167,20 @@ object Encoders {
     )
   }
 
+  /**
+   * An encoder for 2-ary tuples.
+   * @since 1.6.0
+   */
   def tuple[T1, T2](
       e1: Encoder[T1],
       e2: Encoder[T2]): Encoder[(T1, T2)] = {
     ExpressionEncoder.tuple(encoderFor(e1), encoderFor(e2))
   }
 
+  /**
+   * An encoder for 3-ary tuples.
+   * @since 1.6.0
+   */
   def tuple[T1, T2, T3](
       e1: Encoder[T1],
       e2: Encoder[T2],
@@ -133,6 +188,10 @@ object Encoders {
     ExpressionEncoder.tuple(encoderFor(e1), encoderFor(e2), encoderFor(e3))
   }
 
+  /**
+   * An encoder for 4-ary tuples.
+   * @since 1.6.0
+   */
   def tuple[T1, T2, T3, T4](
       e1: Encoder[T1],
       e2: Encoder[T2],
@@ -141,6 +200,10 @@ object Encoders {
     ExpressionEncoder.tuple(encoderFor(e1), encoderFor(e2), encoderFor(e3), encoderFor(e4))
   }
 
+  /**
+   * An encoder for 5-ary tuples.
+   * @since 1.6.0
+   */
   def tuple[T1, T2, T3, T4, T5](
       e1: Encoder[T1],
       e2: Encoder[T2],