Documentation: coresight: Expand branch broadcast documentation

Now that there is a way of enabling branch broadcast via perf, mention
the possible use cases and known limitations.

Signed-off-by: James Clark <james.clark@arm.com>
Reviewed-by: Mike <Leach&lt;mike.leach@linaro.org>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20220511144601.2257870-5-james.clark@arm.com