arm64/sve: Minor clarification of ABI documentation

arm64/sve: Minor clarification of ABI documentation

As suggested by Luis for the SME version of this explicitly say that the
vector length should be extracted from the return value of a set vector
length prctl() with a bitwise and rather than just any old and.

Suggested-by: Luis Machado <Luis.Machado@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20211210184133.320748-4-broonie@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>