Add Filter order to documentation

[MrGeneration]

Documentation currently doesn't hit about the working order of postmaster filters which may confuse admins.

[abrain]

Is there a short answer that you could share here?

Also related: If multiple filters match, does the first or last filter determine the group of the ticket?

[MrGeneration]

If you have technical questions please consult our community: https://community.zammad.org