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

Documentation/arm64/sve.rst

@@ -255,7 +255,7 @@ prctl(PR_SVE_GET_VL)
     vector length change (which would only normally be the case between a
     fork() or vfork() and the corresponding execve() in typical use).
 
-    To extract the vector length from the result, and it with
+    To extract the vector length from the result, bitwise and it with
     PR_SVE_VL_LEN_MASK.
 
     Return value: a nonnegative value on success, or a negative value on error: