docs: ABI: make the KernelVersion field optional

docs: ABI: make the KernelVersion field optional

The KernelVersion field has limited practical value. Git history
provides more accurate tracking of when features were introduced
and target kernel versions often change during development and
merge.

Label it optional.

Signed-off-by: Alison Schofield <alison.schofield@intel.com>
Acked-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250626024101.792073-1-alison.schofield@intel.com