arm64/gcs: Fix outdated ptrace documentation

arm64/gcs: Fix outdated ptrace documentation

The ptrace documentation for GCS was written prior to the implementation of
clone3() when we still blocked enabling of GCS via ptrace. This restriction
was relaxed as part of implementing clone3() support since we implemented
support for the GCS not being managed by the kernel but the documentation
still mentions the restriction. Update the documentation to reflect what
was merged.

We have not yet merged clone3() itself but all the support other than in
clone() itself is there.

Fixes: 7058bf87cd59 ("arm64/gcs: Document the ABI for Guarded Control Stacks")
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20241031-arm64-gcs-doc-disable-v1-1-d7f6ded62046@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>