docs: rust: update description of crates

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

docs/devel/rust.rst

@@ -139,16 +139,22 @@ anymore.
 Writing Rust code in QEMU
 -------------------------
 
-Right now QEMU includes three crates:
+QEMU includes four crates:
 
 * ``qemu_api`` for bindings to C code and useful functionality
 
 * ``qemu_api_macros`` defines several procedural macros that are useful when
   writing C code
 
-* ``pl011`` (under ``rust/hw/char/pl011``) is the sample device that is being
-  used to further develop ``qemu_api`` and ``qemu_api_macros``.  It is a functional
-  replacement for the ``hw/char/pl011.c`` file.
+* ``pl011`` (under ``rust/hw/char/pl011``) and ``hpet`` (under ``rust/hw/timer/hpet``)
+  are sample devices that demonstrate ``qemu_api`` and ``qemu_api_macros``, and are
+  used to further develop them.  These two crates are functional\ [#issues]_ replacements
+  for the ``hw/char/pl011.c`` and ``hw/timer/hpet.c`` files.
+
+.. [#issues] The ``pl011`` crate is synchronized with ``hw/char/pl011.c``
+   as of commit 02b1f7f61928.  The ``hpet`` crate is synchronized as of
+   commit f32352ff9e.  Both are lacking tracing functionality; ``hpet``
+   is also lacking support for migration.
 
 This section explains how to work with them.