raft: atomic access alignment

The relevant structures are properly aligned, however, there is no comment
highlighting the need to keep it aligned as is present elsewhere in the
codebase.

Adding note to keep alignment, in line with similar comments in the codebase.

raft/node.go

@@ -38,7 +38,7 @@ var (
 // SoftState provides state that is useful for logging and debugging.
 // The state is volatile and does not need to be persisted to the WAL.
 type SoftState struct {
-	Lead      uint64
+	Lead      uint64 // must use atomic operations to access; keep 64-bit aligned.
 	RaftState StateType
 }