[SPARK-23062][SQL] Improve EXCEPT documentation

## What changes were proposed in this pull request?

Make the default behavior of EXCEPT (i.e. EXCEPT DISTINCT) more
explicit in the documentation.

R/pkg/R/DataFrame.R

@@ -2853,7 +2853,7 @@ setMethod("intersect",
 #' except
 #'
 #' Return a new SparkDataFrame containing rows in this SparkDataFrame
-#' but not in another SparkDataFrame. This is equivalent to \code{EXCEPT} in SQL.
+#' but not in another SparkDataFrame. This is equivalent to \code{EXCEPT DISTINCT} in SQL.
 #'
 #' @param x a SparkDataFrame.
 #' @param y a SparkDataFrame.

python/pyspark/sql/dataframe.py

@@ -1364,7 +1364,8 @@ def subtract(self, other):
         """ Return a new :class:`DataFrame` containing rows in this frame
         but not in another frame.
 
-        This is equivalent to `EXCEPT` in SQL.
+        This is equivalent to `EXCEPT DISTINCT` in SQL.
+
         """
         return DataFrame(getattr(self._jdf, "except")(other._jdf), self.sql_ctx)
 

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

@@ -1902,7 +1902,7 @@ class Dataset[T] private[sql](
 
   /**
    * Returns a new Dataset containing rows in this Dataset but not in another Dataset.
-   * This is equivalent to `EXCEPT` in SQL.
+   * This is equivalent to `EXCEPT DISTINCT` in SQL.
    *
    * @note Equality checking is performed directly on the encoded representation of the data
    * and thus is not affected by a custom `equals` function defined on `T`.