docs/system: Deprecate raspi2/raspi3 machine aliases

docs/system: Deprecate raspi2/raspi3 machine aliases

Since commit aa35ec2213b ("hw/arm/raspi: Use more specific
machine names") the raspi2/raspi3 machines have been renamed
as raspi2b/raspi3b.

Note, rather than the raspi3b, the raspi3ap introduced in
commit 5be94252d34 ("hw/arm/raspi: Add the Raspberry Pi 3
model A+") is a closer match to what QEMU models, but only
provides 512 MB of RAM.

As more Raspberry Pi 2/3 models are emulated, in order
to avoid confusion, deprecate the raspi2/raspi3 machine
aliases.

ACKed-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20201120173953.2539469-2-f4bug@amsat.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>