[minor doc] Add exploratory data analysis warning for DataFrame.stat.freqItem API

Author: Reynold Xin <rxin@databricks.com>

Closes #6569 from rxin/freqItemsWarning and squashes the following commits:

7eec145 [Reynold Xin] [minor doc] Add exploratory data analysis warning for DataFrame.stat.freqItem API.

python/pyspark/sql/dataframe.py

@@ -1170,6 +1170,9 @@ class DataFrame(object):
         "http://dx.doi.org/10.1145/762471.762473, proposed by Karp, Schenker, and Papadimitriou".
         :func:`DataFrame.freqItems` and :func:`DataFrameStatFunctions.freqItems` are aliases.
 
+        This function is meant for exploratory data analysis, as we make no guarantee about the
+        backward compatibility of the schema of the resulting DataFrame.
+
         :param cols: Names of the columns to calculate frequent items for as a list or tuple of
             strings.
         :param support: The frequency with which to consider an item 'frequent'. Default is 1%.

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

@@ -97,6 +97,9 @@ final class DataFrameStatFunctions private[sql](df: DataFrame) {
    * [[http://dx.doi.org/10.1145/762471.762473, proposed by Karp, Schenker, and Papadimitriou]].
    * The `support` should be greater than 1e-4.
    *
+   * This function is meant for exploratory data analysis, as we make no guarantee about the
+   * backward compatibility of the schema of the resulting [[DataFrame]].
+   *
    * @param cols the names of the columns to search frequent items in.
    * @param support The minimum frequency for an item to be considered `frequent`. Should be greater
    *                than 1e-4.
@@ -114,6 +117,9 @@ final class DataFrameStatFunctions private[sql](df: DataFrame) {
    * [[http://dx.doi.org/10.1145/762471.762473, proposed by Karp, Schenker, and Papadimitriou]].
    * Uses a `default` support of 1%.
    *
+   * This function is meant for exploratory data analysis, as we make no guarantee about the
+   * backward compatibility of the schema of the resulting [[DataFrame]].
+   *
    * @param cols the names of the columns to search frequent items in.
    * @return A Local DataFrame with the Array of frequent items for each column.
    *
@@ -128,6 +134,9 @@ final class DataFrameStatFunctions private[sql](df: DataFrame) {
    * frequent element count algorithm described in
    * [[http://dx.doi.org/10.1145/762471.762473, proposed by Karp, Schenker, and Papadimitriou]].
    *
+   * This function is meant for exploratory data analysis, as we make no guarantee about the
+   * backward compatibility of the schema of the resulting [[DataFrame]].
+   *
    * @param cols the names of the columns to search frequent items in.
    * @return A Local DataFrame with the Array of frequent items for each column.
    *
@@ -143,6 +152,9 @@ final class DataFrameStatFunctions private[sql](df: DataFrame) {
    * [[http://dx.doi.org/10.1145/762471.762473, proposed by Karp, Schenker, and Papadimitriou]].
    * Uses a `default` support of 1%.
    *
+   * This function is meant for exploratory data analysis, as we make no guarantee about the
+   * backward compatibility of the schema of the resulting [[DataFrame]].
+   *
    * @param cols the names of the columns to search frequent items in.
    * @return A Local DataFrame with the Array of frequent items for each column.
    *