Clarify documentation for json parsing

Documentation does not currently mention that is_json accepts mimetypes that are not strictly application/json.

flask/wrappers.py

@@ -97,7 +97,7 @@ class Request(RequestBase):
 
     @property
     def json(self):
-        """If the mimetype is :mimetype:`application/json` this will contain the
+        """If self.is_json would return true, this will contain the
         parsed JSON data.  Otherwise this will be ``None``.
 
         The :meth:`get_json` method should be used instead.
@@ -124,11 +124,10 @@ class Request(RequestBase):
 
     def get_json(self, force=False, silent=False, cache=True):
         """Parses the incoming JSON request data and returns it.  By default
-        this function will return ``None`` if the mimetype is not
-        :mimetype:`application/json` but this can be overridden by the
-        ``force`` parameter. If parsing fails the
-        :meth:`on_json_loading_failed` method on the request object will be
-        invoked.
+        this function will return ``None`` if self.is_json would return false,
+        but this can be overridden by the ``force`` parameter. If parsing
+        fails, the :meth:`on_json_loading_failed` method on the request object
+        will be invoked.
 
         :param force: if set to ``True`` the mimetype is ignored.
         :param silent: if set to ``True`` this method will fail silently