> From: Philipp Stephani <p.stephani2@gmail.com>
> Date: Sun, 28 Jan 2018 20:59:35 +0000
> Cc: 30163@debbugs.gnu.org
>
>  C-h f mod-test-sum RET
>
>    -> "mod-test-sum is ."
>
>  Normally one sees something like "is a built-in function in foo.c." etc.
>
> I've attached a patch.

LGTM, thanks.