The Filesystem Heirarchy Standard does not give clear guidance (at least to me) on placement of files under these circumstances. However, interpreting what it has to say about things in general that seem to also apply here, I would place such documents in one of the following places (in order of preference):
- /home/[department or category]/share
Consideration of ease of managing access rights should be given to how you plan to further subdivide the structure.
A couple of commenters have mentioned /srv and one could make the argument that your files could be placed under /srv/samba. However, I disagree if the files may be made available via other services or locally to the host system.
Granted, these are my interpretations and recommendations. If you want better guidance, you should consult the FHS mailing list.