Add a comment to clarify how to handle best-effort backwards
compatibility for LANDLOCK_ACCESS_FS_REFER.

The "refer" access is special because these operations are always
forbidden in ABI 1, unlike most other operations, which are permitted
when using Landlock ABI levels where they are not supported yet.
Signed-off-by: Günther Noack <gnoack3000@gmail.com>
Link: https://lore.kernel.org/r/20221107181651.4555-1-gnoack3000@gmail.comSigned-off-by: Mickaël Salaün <mic@digikod.net>