[SQL] [MINOR] Adds @deprecated Scaladoc entry for SchemaRDD

Author: Cheng Lian <lian@databricks.com>

Closes #6529 from liancheng/schemardd-deprecation-fix and squashes the following commits:

49765c2 [Cheng Lian] Adds @deprecated Scaladoc entry for SchemaRDD

sql/core/src/main/scala/org/apache/spark/sql/package.scala

@@ -44,6 +44,7 @@ package object sql {
 
   /**
    * Type alias for [[DataFrame]]. Kept here for backward source compatibility for Scala.
+   * @deprecated As of 1.3.0, replaced by `DataFrame`.
    */
   @deprecated("1.3.0", "use DataFrame")
   type SchemaRDD = DataFrame