Documentation: kvm: new UAPI for handling SEA

Documentation: kvm: new UAPI for handling SEA

Document the new userspace-visible features and APIs for handling
synchronous external abort (SEA)
- KVM_CAP_ARM_SEA_TO_USER: How userspace enables the new feature.
- KVM_EXIT_ARM_SEA: exit userspace gets when it needs to handle SEA
  and what userspace gets while taking the SEA.

Signed-off-by: Jiaqi Yan <jiaqiyan@google.com>
Link: https://msgid.link/20251013185903.1372553-4-jiaqiyan@google.com
[ oliver: make documentation concise, remove implementation detail ]
Signed-off-by: Oliver Upton <oupton@kernel.org>