Documentation: Better documentation for the in-place sync

Not sure how much of this is superceeded when 1.0 comes out with its default of in-place import. However, the documentation at https://docs.funkwhale.audio/admin/importing-music.html should be changed to reflect that the command: export LIBRARY_ID="<your_libary_id>" python api/manage.py import_files $LIBRARY_ID "/srv/funkwhale/data/music/" --recursive --noinput should be run as the funkwhale user. So the sudo -u funkwhale should be added.

Additionally, may wish to either have the command automatically use the right pip env via:

sudo -u funkwhale /srv/funkwhale/virtualenv/bin/python /srv/funkwhale/api/manage.py import_files etc.

or give instructions in activating the venv.

Cheers.