Fix broken IResourcePreview docstring

Not spell-checked, not following CKAN's docstring guidelines, contained
Sphinx errors. I also rewrote it for clarity.

ckan/plugins/interfaces.py

@@ -196,33 +196,40 @@ def notify(self, resource):
 
 
 class IResourcePreview(Interface):
-    """
-    Hook into the resource previews in helpers.py. This lets you
-    create custom previews for example for xml files.
-    """
+    '''Add custom data previews for resource file-types.
 
+    '''
     def can_preview(self, data_dict):
-        '''
-        Returns info on whether the plugin can preview the resource.
+        '''Return info on whether the plugin can preview the resource.
 
-        This can be done in two ways.
-        The old way is to just return True or False.
-        The new way is to return a dict with the following
-        {
-            'can_preview': bool - if the extension can preview the resource
-            'fixable': string - if the extension cannot preview but could for
-                                example if the resource_proxy was enabled.
-            'quality': int - how good the preview is 1-poor, 2-average, 3-good
-                          used if multiple extensions can preview
-        }
+        This can be done in two ways:
 
-        The ``data_dict`` contains the resource and the package.
+        1. The old way is to just return ``True`` or ``False``.
+
+        2. The new way is to return a dict with  three keys:
+
+           ``'can_preview'`` (``boolean``)
+             ``True`` if the extension can preview the resource.
+
+           ``'fixable'`` (``string``)
+             A string explaining how preview for the resource could be enabled,
+             for example if the ``resource_proxy`` plugin was enabled.
+
+           ``'quality'`` (``int``)
+             How good the preview is: ``1`` (poor), ``2`` (average) or
+             ``3`` (good). When multiple preview extensions can preview the
+             same resource, this is used to determine which extension will
+             be used.
+
+        :param data_dict: the resource to be previewed and the dataset that it
+          belongs to.
+        :type data_dict: dictionary
+
+        Make sure to check the ``on_same_domain`` value of the resource or the
+        url if your preview requires the resource to be on the same domain
+        because of the same-origin policy.  To find out how to preview
+        resources that are on a different domain, read :ref:`resource-proxy`.
 
-        Make sure to ckeck the ``on_same_domain`` value of the
-        resource or the url if your preview requires the resource to be on
-        the same domain because of the same origin policy.
-        To find out how to preview resources that are on a
-        different domain, read :ref:`resource-proxy`.
         '''
 
     def setup_template_variables(self, context, data_dict):