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

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

Add some more general explanation of hardware vs virtual timers.

This work was funded through GitHub Sponsors.

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

Author: projectgus

docs/library/machine.Timer.rst

@@ -11,6 +11,10 @@ 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).
 
+Many MicroPython ports also support support "virtual timers" which are managed
+in software. These are generally more flexible, and multiple virtual timers can
+be allocated at once.
+
 See discussion of :ref:`important constraints <machine_callbacks>` on
 Timer callbacks.
 
@@ -30,11 +34,20 @@ Constructors
 
 .. class:: Timer(id, /, ...)
 
-   Construct a new timer object of the given ``id``. ``id`` of -1 constructs a
-   virtual timer (if supported by a board).
+   Construct a new timer object with the given ``id``.
+
+   If the port supports virtual timers then the ``id`` parameter can be omitted
+   or set to value ``-1`` to construct a virtual timer. It's recommended to use
+   virtual timers if the port supports this, unless you need very precise
+   sub-millisecond callback timing (with hard interrupt context).
+
+   To construct a hardware timer, the ``id`` parameter must be set to a
+   non-negative integer value. Most microcontroller ports support hardware
+   timers, except the Zephyr and RP2 ports which support virtual timers only.
+
    ``id`` shall not be passed as a keyword argument.
 
-   See ``init`` for parameters of initialisation.
+   Any additional parameters are handled the same as :func:`Timer.init()`.
 
 Methods
 -------