wal: add comment

wal/wal.go

@@ -94,6 +94,7 @@ func Create(dirpath string) (*WAL, error) {
 }
 
 // OpenAtIndex opens the WAL at the given index.
+// There MUST be an entry with given index written to WAL before.
 // The returned WAL is ready to read and the first record will be the given
 // index. The WAL cannot be appended to before reading out all of its
 // previous records.