Added comment in SfxModule with warning about using smart pointers

Change-Id: I1997d41a0a77c7e82e486bcdaee0ea876fdc61ad
Reviewed-on: https://gerrit.libreoffice.org/25999Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>