bug#30163: describe-function and modules

[Glenn Morris]

Package: emacs
Version: 26.0.91
Severity: minor

In a --with-modules build:

./src/emacs -Q -l ./test/data/emacs-module/mod-test.so

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.