message: Update CIDER docs now that it is used for main composer as well
diff: The CIDER editor is a custom editor written for Riot.
  Most of the code can be found in the `/editor/` directory of the `matrix-react-sdk` project.
  -It is used to power the composer to edit messages,
  -and will soon be used as the main composer to send messages as well.
  +It is used to power the composer main composer (both to send and edit messages), and might be used for other usecases where autocomplete is desired (invite box, ...).
  ## High-level overview.
  