Update doc strings for register_trigger(...) (#239)

* Update flags.py

* Update flags.py

* Update flags.py

* Update flags.py

charms/reactive/flags.py

@@ -154,11 +154,12 @@ def register_trigger(when=None, when_not=None, set_flag=None, clear_flag=None, c
 
     Note: Flag triggers are handled at the same time that the given flag is set.
 
-    :param str when: Flag to trigger on when it is set.
-    :param str when_not: Flag to trigger on when it is cleared.
-    :param str set_flag: If given, this flag will be set when the relevant flag is changed.
-    :param str clear_flag: If given, this flag will be cleared when the relevant flag is changed.
-    :param str callback: If given, this callback will be invoked when the relevant flag is changed.
+    :param Optional[str] when: Flag to trigger on when it is set.
+    :param Optional[str] when_not: Flag to trigger on when it is cleared.
+    :param Optional[str] set_flag: If given, this flag will be set when the relevant flag is changed.
+    :param Optional[str] clear_flag: If given, this flag will be cleared when the relevant flag is changed.
+    :param Optional[Callable[[], Any]] callback:
+                       If given, this callback will be invoked when the relevant flag is changed.
 
     Note: Exactly one of either `when` or `when_not`, and at least one of
     `set_flag`, `clear_flag`, or `callback` must be provided.