doc: Follow-up commit to 407ebeaa1.

Following some discussion with Ludovic (see:
https://lists.gnu.org/archive/html/guix-patches/2019-08/msg00506.html), it is
better advice to generally recommend the use of `program-file' for any usage
of modules, not just for those which define syntax.

* doc/guix.texi (Scheduled Job Execution): Drop the following text: "that
defines syntax (macros)".