docs/uselect: Document one-shot polling mode.

docs/library/uselect.rst

@@ -66,12 +66,18 @@ Methods
 
       Tuples returned may contain more than 2 elements as described above.
 
-.. method:: poll.ipoll([timeout])
+.. method:: poll.ipoll(timeout=-1, flags=0)
 
    Like :meth:`poll.poll`, but instead returns an iterator which yields
-   callee-owned tuples. This function provides efficient, allocation-free
+   `callee-owned tuples`. This function provides efficient, allocation-free
    way to poll on streams.
 
+   If *flags* is 1, one-shot behavior for events is employed: streams for
+   which events happened, event mask will be automatically reset (equivalent
+   to ``poll.modify(obj, 0)``), so new events for such a stream won't be
+   processed until new mask is set with `poll.modify()`. This behavior is
+   useful for asynchronous I/O schedulers.
+
    .. admonition:: Difference to CPython
       :class: attention