PM: hibernate: fix docs for ioctls that return loff_t via pointer

Correctly document how the SNAPSHOT_GET_IMAGE_SIZE and
SNAPSHOT_AVAIL_SWAP_SIZE ioctls return their result.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>