docs/library: Add description of "index" parameter to uos.dupterm().

docs/library/uos.rst

@@ -89,8 +89,22 @@ Functions
    Return a bytes object with n random bytes. Whenever possible, it is
    generated by the hardware random number generator.
 
-.. function:: dupterm(stream_object)
+.. function:: dupterm(stream_object, index=0)
 
-   Duplicate or switch MicroPython terminal (the REPL) on the passed stream-like
-   object. The given object must implement the ``readinto()`` and ``write()``
-   methods. If ``None`` is passed, previously set redirection is cancelled.
+   Duplicate or switch the MicroPython terminal (the REPL) on the given stream-like
+   object. The *stream_object* argument must implement the ``readinto()`` and
+   ``write()`` methods.  The stream should be in non-blocking mode and
+   ``readinto()`` should return ``None`` if there is no data available for reading.
+
+   After calling this function all terminal output is repeated on this stream,
+   and any input that is available on the stream is passed on to the terminal input.
+
+   The *index* parameter should be a non-negative integer and specifies which
+   duplication slot is set.  A given port may implement more than one slot (slot 0
+   will always be available) and in that case terminal input and output is
+   duplicated on all the slots that are set.
+
+   If ``None`` is passed as the *stream_object* then duplication is cancelled on
+   the slot given by *index*.
+
+   The function returns the previous stream-like object in the given slot.