List description can't include newlines.
The description is included in the mail headers verbatim (probably as part of the List-Id field), and a newline breaks RFC 5322 conformance. I think the right way to handle this is to restructure such descriptions to look like folded headers. My taste would be to (1) trim all trailing linebreaks, then (2) insert 3 spaces after any remaining newlines. The logic for this is that most likely the line breaks are also sentence breaks so MUAs that display such headers verbatim will look like the author probably intended, and MUAs that unfold the header into one logical line, then rewrap to fit the window, will have two spaces there as in the traditional sentence division (note, there may be no period at the end of the preceding line in many cases, the extra space will emphasize the division into sentences).
Alternatively we could just document that descriptions must be a single physical line. But see hyperkitty#100.
for an example of what happens.