Document automatic mailing lists in the admin UI

One-sentence description

Add a few lines of text to the admin interface that describe what the automatic mailing lists do.

Spin-off of #388 (closed)

Why?

Nobody reads code, nobody properly communicates this information to the next boards.

Current implementation

People know(tm)

Desired implementation

Include some text in the admin UI describing leden@ etc.

Edited by Ghost User
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