Add missing documentation of peer_device parameter to Kernel doc.

These parameters were added in commit 8164dd6c ("drbd: Add peer
device parameter to whole-bitmap I/O handlers")

Flagged by W=1 builds.
Signed-off-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240723-drbd-doc-v1-1-a04d9b7a9688@kernel.orgSigned-off-by: Jens Axboe <axboe@kernel.dk>