[Bug target/79869] i18n: document placeholders for translators

[roland.illig at gmx dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79869

--- Comment #1 from Roland Illig <roland.illig at gmx dot de> ---
ping? Two years later, and I still don't know how to translate this string into
proper German.