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 <[e-mail]>
Acked-by: Dan Williams <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/ABI/README

···
 
 What:		Short description of the interface
 Date:		Date created
-KernelVersion:	Kernel version this feature first showed up in.
+KernelVersion:	(Optional) Kernel version this feature first showed up in.
+		Note: git history often provides more accurate version
+		info, so this field may be omitted.
 Contact:	Primary contact for this interface (may be a mailing list)
 Description:	Long description of the interface and how to use it.
 Users:		All users of this interface who wish to be notified when