docs/about/deprecated: Document RISC-V "pmu-num" deprecation

This has been replaced by a "pmu-mask" property that provides much more
flexibility.

Signed-off-by: Rob Bradford <rbradford@rivosinc.com>
Acked-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Message-ID: <20231031154000.18134-6-rbradford@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

docs/about/deprecated.rst

@@ -413,6 +413,18 @@ Specifying the iSCSI password in plain text on the command line using the
 used instead, to refer to a ``--object secret...`` instance that provides
 a password via a file, or encrypted.
 
+CPU device properties
+'''''''''''''''''''''
+
+``pmu-num=n`` on RISC-V CPUs (since 8.2)
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+In order to support more flexible counter configurations this has been replaced
+by a ``pmu-mask`` property. If set of counters is continuous then the mask can
+be calculated with ``((2 ^ n) - 1) << 3``. The least significant three bits
+must be left clear.
+
+
 Backwards compatibility
 -----------------------