docs/library/machine.Timer: Explain the id parameter in more detail.

As noted in discussion on PR #18263, the id parameter is optional on ports
that support virtual timers.

Add some more general explanation of hardware vs virtual timers, and remove
redundant documentation about timer callbacks in favour of the isr_rules
page.

This work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

docs/library/machine.TimerWiPy.rst

@@ -18,16 +18,6 @@ defines a baseline operation of executing a callback with a given period
 (or once after some delay), and allow specific boards to define more
 non-standard behaviour (which thus won't be portable to other boards).
 
-See discussion of :ref:`important constraints <machine_callbacks>` on
-Timer callbacks.
-
-.. note::
-
-    Memory can't be allocated inside irq handlers (an interrupt) and so
-    exceptions raised within a handler don't give much information.  See
-    :func:`micropython.alloc_emergency_exception_buf` for how to get around this
-    limitation.
-
 Constructors
 ------------
 
@@ -134,6 +124,9 @@ Methods
           ``TimerWiPy.ONE_SHOT``. In the case that mode is ``TimerWiPy.PWM`` then trigger must be equal to
           ``TimerWiPy.MATCH``.
 
+    Note that callback handlers are hard interrupts, and the constraints described in :ref:`isr_rules`
+    apply when they are executed.
+
     Returns a callback object.
 
 .. method:: timerchannel.freq([value])