docs/uselect: Describe more aspects of poll.register/modify behavior.

E.g., register() can be called again for the same object, while modify()
will raise exception if object was not register()ed before.

docs/library/uselect.rst

@@ -45,13 +45,18 @@ Methods
 
    *eventmask* defaults to ``uselect.POLLIN | uselect.POLLOUT``.
 
+   It is OK to call this function multiple times for the same *obj*.
+   Successive calls will update *obj*'s eventmask to the value of
+   *eventmask* (i.e. will behave as `modify()`).
+
 .. method:: poll.unregister(obj)
 
    Unregister *obj* from polling.
 
 .. method:: poll.modify(obj, eventmask)
 
-   Modify the *eventmask* for *obj*.
+   Modify the *eventmask* for *obj*. If *obj* is not registered, `OSError`
+   is raised with error of ENOENT.
 
 .. method:: poll.poll(timeout=-1)