Update using infos from kmail docbooks added by Scarlett

* Change book to article
* Make application name translatable in docbook

This docbooks does not show up in KHelpcenter navigation tree, but as it can't be started standalone that is ok?

Questions + missing infos:

* What happens if a) kmail not running b) kmail in offline mode c) no network connection or 4) computer not running
when a scheduled message should be send by the agent?

* Is it possible to edit content, sender, receiver, subject of a scheduled message and how?

* waht to to whith the infos in kmail docbooks? replace with links to this docbook?

P.S.
Similar updates should be made for akonadi_archivemail_agent + akonadi_folderarchive_agents as well