docs: Document the cross-port Timer hard= option.

Update the main machine.Timer specification, and any references to
hard/soft interrupts in port-specific documentation. There is a separate
copy of the machine.Timer documentation for the pyboard, so update that
too to keep everything consistent.

Signed-off-by: Chris Webb <chris@arachsys.com>

docs/esp8266/quickref.rst

@@ -108,6 +108,9 @@ with timer ID of -1::
 
 The period is in milliseconds.
 
+By default, timer callbacks are scheduled as soft interrupts on this port.
+Specify ``hard=True`` to run them in hard interrupt context instead.
+
 Pins and GPIO
 -------------