migrating `mail_location` deserves some more information

[wolegis]

IMHO the line

mail_location setting & mail userdb field | Split into multiple mail_* settings.

in Upgrading Dovecot CE from 2.3 to 2.4 deserves a separate section explaining the migration in more detail.

In my case it took considerable time and several trials and errors to figure out that previous

mail_home = /srv/vmail/%d/%u
mail_location = sdbox:~/mail

needed to be transformed to

mail_home = /srv/vmail/%{user|domain}/%{user|username}
mail_driver = sdbox
mail_path = /srv/vmail/%{user|domain}/%{user|username}/mail

In particular, it was not possible to specify mail_path = $SET:mail_home/mail.

[cmouse]

You would use %{home} or ~/ instead of $SET:mail_home.