Fix python docs

Author: lw-lin

python/pyspark/sql/readwriter.py

@@ -318,7 +318,8 @@ def csv(self, path, schema=None, sep=None, encoding=None, quote=None, escape=Non
                                          being read should be skipped. If None is set, it uses
                                          the default value, ``false``.
         :param nullValue: sets the string representation of a null value. If None is set, it uses
-                          the default value, empty string.
+                          the default value, empty string. Since 2.0.1, this ``nullValue`` param
+                          applies to all supported types including the string type.
         :param nanValue: sets the string representation of a non-number value. If None is set, it
                          uses the default value, ``NaN``.
         :param positiveInf: sets the string representation of a positive infinity value. If None

python/pyspark/sql/streaming.py

@@ -485,7 +485,8 @@ def csv(self, path, schema=None, sep=None, encoding=None, quote=None, escape=Non
                                          being read should be skipped. If None is set, it uses
                                          the default value, ``false``.
         :param nullValue: sets the string representation of a null value. If None is set, it uses
-                          the default value, empty string.
+                          the default value, empty string. Since 2.0.1, this ``nullValue`` param
+                          applies to all supported types including the string type.
         :param nanValue: sets the string representation of a non-number value. If None is set, it
                          uses the default value, ``NaN``.
         :param positiveInf: sets the string representation of a positive infinity value. If None