docs/machine.Pin: Document "hard" argument of Pin.irq method.

docs/library/machine.Pin.rst

@@ -179,7 +179,7 @@ Methods
 
    Availability: WiPy.
 
-.. method:: Pin.irq(handler=None, trigger=(Pin.IRQ_FALLING | Pin.IRQ_RISING), \*, priority=1, wake=None)
+.. method:: Pin.irq(handler=None, trigger=(Pin.IRQ_FALLING | Pin.IRQ_RISING), \*, priority=1, wake=None, hard=False)
 
    Configure an interrupt handler to be called when the trigger source of the
    pin is active.  If the pin mode is ``Pin.IN`` then the trigger source is
@@ -213,6 +213,10 @@ Methods
        These values can also be OR'ed together to make a pin generate interrupts in
        more than one power mode.
 
+     - ``hard`` if true a hardware interrupt is used. This reduces the delay
+       between the pin change and the handler being called. Hard interrupt
+       handlers may not allocate memory; see :ref:`isr_rules`.
+
    This method returns a callback object.
 
 Constants