arm64/gcs: Fix outdated ptrace documentation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: 7058bf8 ("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>

Author: broonie

Documentation/arch/arm64/gcs.rst

@@ -204,11 +204,8 @@ When returning from a signal handler:
 * A new regset NT_ARM_GCS is defined for use with PTRACE_GETREGSET and
   PTRACE_SETREGSET.
 
-* Due to the complexity surrounding allocation and deallocation of stacks and
-  lack of practical application it is not possible to enable GCS via ptrace.
-  GCS may be disabled via the ptrace interface.
-
-* Other GCS modes may be configured via ptrace.
+* The GCS mode, including enable and disable, may be configured via ptrace.
+  If GCS is enabled via ptrace no new GCS will be allocated for the thread.
 
 * Configuration via ptrace ignores locking of GCS mode bits.