[Dovecot] trivial documentation errata (1.2.7)

[Frank Cusack]

The default dovecot.conf says

  # Dictionary can be used by some plugins to store key=value lists.
  # Currently this is only used by dict quota backend.

Looks like expire uses dict as well.

The managesieve comments are stale as well, with a reference to
sieve_storage still in there.  Or do they refer to the CMU sieve
implementation?  Anyway it is confusing.

-frank