arm64/sve: Document our actual ABI for clearing registers on syscall

Currently our ABI documentation says that the state of the bits in the Z
registers not shared with the V registers becomes undefined on syscall but
our actual implementation unconditionally clears these bits. Taking
advantage of the flexibility of our documented ABI would be a change in the
observable ABI so there is concern around doing so, instead document the
actual behaviour so that it is more discoverable for userspace programmers
who might be able to take advantage of it and to record our decision about
not changing the kernel ABI.

This makes qemu's user mode implementation buggy since it does not clear
these bits.

Signed-off-by: Mark Brown <broonie@kernel.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20220829162502.886816-3-broonie@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Documentation/arm64/sve.rst

@@ -111,7 +111,7 @@ the SVE instruction set architecture.
 
 * On syscall, V0..V31 are preserved (as without SVE).  Thus, bits [127:0] of
   Z0..Z31 are preserved.  All other bits of Z0..Z31, and all of P0..P15 and FFR
-  become unspecified on return from a syscall.
+  become zero on return from a syscall.
 
 * The SVE registers are not used to pass arguments to or receive results from
   any syscall.