docs/system/target-arm.rst: Remove "many boards are undocumented" note

docs/system/target-arm.rst: Remove "many boards are undocumented" note

We now have at least placeholder documentation for every Arm board,
so we can remove the apologetic note that says that there are
undocumented ones which you can only find out about via the
``--machine help`` option.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20241018141332.942844-8-peter.maydell@linaro.org