Documented gdal DataSource encoding parameter

Thanks Max Demars for the suggestion and Tim Graham for the review.

docs/ref/contrib/gis/gdal.txt

@@ -55,9 +55,9 @@ points, polygons, etc.), as well as the names and types of any
 additional fields (:class:`Field`) of data that may be associated with
 each feature in that layer.
 
-.. class:: DataSource(ds_input)
+.. class:: DataSource(ds_input, [encoding='utf-8'])
 
-   The constructor for ``DataSource`` just a single parameter: the path of
+   The constructor for ``DataSource`` only requires one parameter: the path of
    the file you want to read. However, OGR
    also supports a variety of more complex data sources, including
    databases, that may be accessed by passing a special name string instead
@@ -66,6 +66,11 @@ each feature in that layer.
    instance gives the OGR name of the underlying data source that it is
    using.
 
+   The optional ``encoding`` parameter allows you to
+   specify a non-standard encoding of the strings in the source. This is
+   typically useful when you obtain ``DjangoUnicodeDecodeError`` exceptions
+   while reading field values.
+
    Once you've created your ``DataSource``, you can find out how many
    layers of data it contains by accessing the :attr:`layer_count` property,
    or (equivalently) by using the ``len()`` function.  For information on