reader: add commentary to index lookup failure case in reader_iter_seek.

Author: cmikk

mtbl/reader.c

@@ -416,6 +416,10 @@ reader_iter_seek(void *v,
 	block_iter_seek(it->index_iter, key, len_key);
 
 	if (!block_iter_get(it->index_iter, &ikey, &len_ikey, &ival, &len_ival)) {
+		/* This seek puts us after the last key, so we mark the
+		 * iterator as invalid and return success. The next
+		 * mtbl_iter_next() operation will return mtbl_res_failure.
+		 */
 		it->valid = false;
 		return (mtbl_res_success);
 	}