dm cache policy: fix description of lookup fn

Correct the documented requirement on the return code from dm cache policy
lookup functions stated in the policy module header file.

Signed-off-by: Alasdair G Kergon <[e-mail]>

drivers/md/dm-cache-policy.h

···
 	 *
 	 * Must not block.
 	 *
-	 * Returns 1 iff in cache, 0 iff not, < 0 on error (-EWOULDBLOCK
-	 * would be typical).
+	 * Returns 0 if in cache, -ENOENT if not, < 0 for other errors
+	 * (-EWOULDBLOCK would be typical).
 	 */
 	int (*lookup)(struct dm_cache_policy *p, dm_oblock_t oblock, dm_cblock_t *cblock);