arm64/fp: Clarify effect of setting an unsupported system VL

The documentation for system vector length configuration does not cover all
cases where unsupported values are written, tighten it up.

Reported-by: Edmund Grimley-Evans <edmund.grimley-evans@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Dave Martin <Dave.Martin@arm.com>
Link: https://lore.kernel.org/r/20240124-arm64-sve-sme-doc-v2-3-fe3964fb3c19@kernel.org


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>