docs/rp2: Document the new rp2 Timer hard= option.

Signed-off-by: Chris Webb <[email protected]>

Author: arachsys

docs/rp2/quickref.rst

@@ -135,6 +135,12 @@ Use the :mod:`machine.Timer` class::
     tim = Timer(period=5000, mode=Timer.ONE_SHOT, callback=lambda t:print(1))
     tim.init(period=2000, mode=Timer.PERIODIC, callback=lambda t:print(2))
 
+By default, timer callbacks run as soft IRQs so they can allocate but
+are prone to GC jitter and delays. Pass ``hard=True`` to the ``Timer()``
+constructor or ``init()`` method to run the callback in hard-IRQ context
+instead. This reduces delay and jitter, but see :ref:`isr_rules` for the
+restrictions that apply to hard-IRQ handlers.
+
 
 .. _rp2_Pins_and_GPIO: