message: Clarify presence of `Responder`
diff: //! **Requests** are some of the most complex effects, they represent a question of a component for
  //! another component, for which it eventually expects an answer.
  //!
  -//! A request usually has a `Responder` in its fields, which a handler of a request **must** call at
  +//! A request **must** have a `Responder` field, which a handler of a request **must** call at
  //! some point. Failing to do so will result in a resource leak.
  pub mod announcements;
  