improved text related to na values in io.rst file

Author: OlegShteynbuk

doc/source/io.rst

@@ -225,8 +225,9 @@ NA and Missing Data Handling
 
 na_values : scalar, str, list-like, or dict, default ``None``
   Additional strings to recognize as NA/NaN. If dict passed, specific per-column
-  NA values. By default the following values are interpreted as NaN: See :ref:`na values const
-  <io.navaluesconst>` below.
+  NA values. See :ref:`na values const <io.navaluesconst>` below
+  for a list of the values interpreted as NaN by default.
+
 keep_default_na : boolean, default ``True``
   If na_values are specified and keep_default_na is ``False`` the default NaN
   values are overridden, otherwise they're appended to.
@@ -1033,9 +1034,7 @@ To completely override the default values that are recognized as missing, specif
 .. _io.navaluesconst:
 
 The default ``NaN`` recognized values are ``['-1.#IND', '1.#QNAN', '1.#IND', '-1.#QNAN', '#N/A N/A', '#N/A', 'N/A',
-'n/a', 'NA', '#NA', 'NULL', 'null', 'NaN', '-NaN', 'nan', '-nan', '']``. Although a 0-length string
-``''`` is not included in the default ``NaN`` values list, it is still treated
-as a missing value.
+'n/a', 'NA', '#NA', 'NULL', 'null', 'NaN', '-NaN', 'nan', '-nan', '']``.
 
 .. code-block:: python