Closing

At the top of the 'node summaries' page, I needed to have a description for the categories.

I decided to use the 'description' field of the taxonomies also for that functinality, just like they're used for the categories overview page.
This patch uses the { !-- break -- } delimiter for that just like in normal nodes.
All text _before_ the break will show up in the categories overview.
All text before and after the delimiter will show up in the nodes summary page.
(One deviation for backward compatibility: if there is _no_ delimiter in the text, then _nothing_ will show up in the nodes summary -- so if you only want text there and not in the categories overview, then start with the delimeter at the first character of the description.)

Patch attached.