message: Add a bit of docs for `component` module
diff: //! Components
  //!
  -//! Docs to be written, sorry.
  -
  +//! Components are the building blocks of the whole application, wired together inside a reactor.
  +/// Each component has a unified interface, expressed by the `Component` trait.
  pub(crate) mod small_network;
  pub(crate) mod storage;
  