docs/system: Document the removal of "compat" property for POWER CPUs

This is just an oversight.

Fixes: f518be3 ("target/ppc: Remove "compat" property of server class POWER CPUs")
Cc: groug@kaod.org
Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <161399328834.51902.14269239378658110394.stgit@bahia.lan>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

docs/system/removed-features.rst

@@ -285,6 +285,12 @@ The RISC-V no MMU cpus have been removed. The two CPUs: ``rv32imacu-nommu`` and
 ``rv64imacu-nommu`` can no longer be used. Instead the MMU status can be specified
 via the CPU ``mmu`` option when using the ``rv32`` or ``rv64`` CPUs.
 
+``compat`` property of server class POWER CPUs (removed in 6.0)
+'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
+
+The ``max-cpu-compat`` property of the ``pseries`` machine type should be used
+instead.
+
 System emulator machines
 ------------------------