update docstring on x_module.get_score to match reality

common/lib/xmodule/xmodule/x_module.py

@@ -288,8 +288,20 @@ class XModule(HTMLSnippet):
         return '{}'
 
     def get_score(self):
-        ''' Score the student received on the problem.
-        '''
+        """
+        Score the student received on the problem, or None if there is no
+        score.
+
+        Returns:
+          dictionary
+             {'score': integer, from 0 to get_max_score(),
+              'total': get_max_score()}
+
+          NOTE (vshnayder): not sure if this was the intended return value, but
+          that's what it's doing now.  I suspect that we really want it to just
+          return a number.  Would need to change (at least) capa and
+          modx_dispatch to match if we did that.
+        """
         return None
 
     def max_score(self):