Mention NaN handling in dtype description (pandas-dev#20895)

Author: jowagner

doc/source/io.rst

@@ -168,7 +168,8 @@ General Parsing Configuration
 
 dtype : Type name or dict of column -> type, default ``None``
   Data type for data or columns. E.g. ``{'a': np.float64, 'b': np.int32}``
-  (unsupported with ``engine='python'``). Use `str` or `object` to preserve and
+  (unsupported with ``engine='python'``). Use `str` or `object` together
+  with suitable ``na_values`` settings to preserve and
   not interpret dtype.
 
   .. versionadded:: 0.20.0 support for the Python parser.

pandas/io/parsers.py

@@ -125,7 +125,8 @@
     are duplicate names in the columns.
 dtype : Type name or dict of column -> type, default None
     Data type for data or columns. E.g. {'a': np.float64, 'b': np.int32}
-    Use `str` or `object` to preserve and not interpret dtype.
+    Use `str` or `object` together with suitable `na_values` settings
+    to preserve and not interpret dtype.
     If converters are specified, they will be applied INSTEAD
     of dtype conversion.
 %s