Add additional docstring for the OAuth2Credentials._expires_in() method,

explaining what values are returned when token_expiry is in the past or
None.

oauth2client/client.py

@@ -629,7 +629,14 @@ class OAuth2Credentials(Credentials):
     self.store = store
 
   def _expires_in(self):
-    """Return the number of seconds until this token expires."""
+    """Return the number of seconds until this token expires.
+    
+    If token_expiry is in the past, this method will return 0, meaning the
+    token has already expired.
+    If token_expiry is None, this method will return None. Note that returning
+    0 in such a case would not be fair: the token may still be valid;
+    we just don't know anything about it. 
+    """
     if self.token_expiry:
       now = datetime.datetime.utcnow()
       if self.token_expiry > now: