[MINOR][DOCS][PYTHON] Adding missing boolean type for replacement value in fillna

## What changes were proposed in this pull request?

Currently pyspark Dataframe.fillna API supports boolean type when we pass dict, but it is missing in documentation.

## How was this patch tested?
>>> spark.createDataFrame([Row(a=True),Row(a=None)]).fillna({"a" : True}).show()
+----+
|   a|
+----+
|true|
|true|
+----+

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: Srinivasa Reddy Vundela <vsr@cloudera.com>

Closes #17688 from vundela/fillna_doc_fix.

python/pyspark/sql/dataframe.py

@@ -1247,7 +1247,7 @@ class DataFrame(object):
             Value to replace null values with.
             If the value is a dict, then `subset` is ignored and `value` must be a mapping
             from column name (string) to replacement value. The replacement value must be
-            an int, long, float, or string.
+            an int, long, float, boolean, or string.
         :param subset: optional list of column names to consider.
             Columns specified in subset that do not have matching data type are ignored.
             For example, if `value` is a string, and subset contains a non-string column,

python/pyspark/sql/tests.py

@@ -1711,6 +1711,10 @@ class SQLTests(ReusedPySparkTestCase):
         self.assertEqual(row.age, None)
         self.assertEqual(row.height, None)
 
+        # fillna with dictionary for boolean types
+        row = self.spark.createDataFrame([Row(a=None), Row(a=True)]).fillna({"a": True}).first()
+        self.assertEqual(row.a, True)
+
     def test_bitwise_operations(self):
         from pyspark.sql import functions
         row = Row(a=170, b=75)