docs/library/machine.Pin: Add PULL_HOLD constant to possible pin pulls.

As already mentioned in the docs, not all constants may be available on all
ports, so this is optional to implement.

docs/library/machine.Pin.rst

@@ -235,6 +235,7 @@ not all constants are available on all ports.
 
 .. data:: Pin.PULL_UP
           Pin.PULL_DOWN
+          Pin.PULL_HOLD
 
    Selects whether there is a pull up/down resistor.  Use the value
    ``None`` for no pull.