message: Document the use of data: Some(b"") for clearing data
diff: @@ -292,7 +292,9 @@ to the caller of `execute` (either the client or another transaction), just as
  if the original `execute` and returned `data: Some(b"better idea")`. If `reply`
  returns `data: None`, it will not modify any previously set data state. If there
  are multiple submessages all setting this, only the last one is used (they all
  -overwrite any previous `data` value).
  +overwrite any previous `data` value). As a consequence, you can use
  +`data: Some(b"")` to clear previously set data. This will be represented as a
  +JSON string instead of `null` and handled as any other `Some` value.
  #### Order and Rollback
  