[SPARK-20041][DOC] Update docs for NaN handling in approxQuantile

## What changes were proposed in this pull request?
Update docs for NaN handling in approxQuantile.

## How was this patch tested?
existing tests.

Author: Zheng RuiFeng <ruifengz@foxmail.com>

Closes #17369 from zhengruifeng/doc_quantiles_nan.

R/pkg/R/stats.R

@@ -149,7 +149,8 @@ setMethod("freqItems", signature(x = "SparkDataFrame", cols = "character"),
 #' This method implements a variation of the Greenwald-Khanna algorithm (with some speed
 #' optimizations). The algorithm was first present in [[http://dx.doi.org/10.1145/375663.375670
 #' Space-efficient Online Computation of Quantile Summaries]] by Greenwald and Khanna.
-#' Note that rows containing any NA values will be removed before calculation.
+#' Note that NA values will be ignored in numerical columns before calculation. For
+#'   columns only containing NA values, an empty list is returned.
 #'
 #' @param x A SparkDataFrame.
 #' @param cols A single column name, or a list of names for multiple columns.

mllib/src/main/scala/org/apache/spark/ml/feature/QuantileDiscretizer.scala

@@ -93,8 +93,8 @@ private[feature] trait QuantileDiscretizerBase extends Params
  * are too few distinct values of the input to create enough distinct quantiles.
  *
  * NaN handling:
- * NaN values will be removed from the column during `QuantileDiscretizer` fitting. This will
- * produce a `Bucketizer` model for making predictions. During the transformation,
+ * null and NaN values will be ignored from the column during `QuantileDiscretizer` fitting. This
+ * will produce a `Bucketizer` model for making predictions. During the transformation,
  * `Bucketizer` will raise an error when it finds NaN values in the dataset, but the user can
  * also choose to either keep or remove NaN values within the dataset by setting `handleInvalid`.
  * If the user chooses to keep NaN values, they will be handled specially and placed into their own

python/pyspark/sql/dataframe.py

@@ -1384,7 +1384,8 @@ class DataFrame(object):
         Space-efficient Online Computation of Quantile Summaries]]
         by Greenwald and Khanna.
 
-        Note that rows containing any null values will be removed before calculation.
+        Note that null values will be ignored in numerical columns before calculation.
+        For columns only containing null values, an empty list is returned.
 
         :param col: str, list.
           Can be a single column name, or a list of names for multiple columns.