docs/system/arm/cpu-features.rst: Fix formatting

docs/system/arm/cpu-features.rst: Fix formatting

The markup for the Arm CPU feature documentation is incorrect,
and results in the HTML not rendering correctly -- the first
line of each description is rendered in boldface as if it
were part of the option name.

Reformat to match the styling used in cpu-models-x86.rst.inc.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1479
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20230316105808.1414003-1-peter.maydell@linaro.org
Reviewed-by: Cornelia Huck <cohuck@redhat.com>