[Bug target/79871] i18n: document placeholders for translators

[roland.illig at gmx dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79871

--- Comment #2 from Roland Illig <roland.illig at gmx dot de> ---
Thanks for the explanation. This leaves open the %s.

Looking further in the code, I can see that %s is either the string "constant"
or the string "lane". In other diagnostics, these words are usually translated
(e.g. Konstante and Spur in German). These strings are not translated in this
diagnostic, which means the German GCC users will be presented with something
like this:

file.c:13: error: Konstante hat keine Adresse
file.c:17: error: constant 13 außerhalb des gültigen Bereichs 0 - 9

It is weird to have the word "constant" translated in one case but not in the
other. Therefore it must not be passed as a placeholder.