docs/library/machine.Timer.rst: Document 'id' as positional-only arg.

Signed-off-by: Michael Buesch <m@bues.ch>

docs/library/machine.Timer.rst

@@ -27,11 +27,12 @@ instead of this class.
 Constructors
 ------------
 
-.. class:: Timer(id, ...)
+.. class:: Timer(id, /, ...)
 
-   Construct a new timer object of the given id. Id of -1 constructs a
+   Construct a new timer object of the given ``id``. ``id`` of -1 constructs a
    virtual timer (if supported by a board).
-   
+   ``id`` shall not be passed as a keyword argument.
+
    See ``init`` for parameters of initialisation.
 
 Methods