docs/library/machine.Timer.rst: Add mention of constructor arguments.

docs/library/machine.Timer.rst

@@ -31,6 +31,8 @@ Constructors
 
    Construct a new timer object of the given id. Id of -1 constructs a
    virtual timer (if supported by a board).
+   
+   See ``init`` for parameters of initialisation.
 
 Methods
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