landlock: Improve kernel-doc "Return:" section consistency

landlock: Improve kernel-doc "Return:" section consistency

The canonical kernel-doc form is "Return:" (singular, without trailing
"s").  Normalize all existing "Returns:" occurrences across the Landlock
source tree to the canonical form.

Also fix capitalization for consistency.  Balance descriptions to
describe all possible returned values.

Consolidate bullet-point return descriptions into inline text for
functions with simple two-value or three-value returns for consistency.

Cc: Günther Noack <gnoack@google.com>
Reviewed-by: Günther Noack <gnoack3000@gmail.com>
Link: https://lore.kernel.org/r/20260304193134.250495-3-mic@digikod.net
Signed-off-by: Mickaël Salaün <mic@digikod.net>