add info text to related assemblies form

src/backoffice/locale/de/LC_MESSAGES/django.po

src/backoffice/locale/en/LC_MESSAGES/django.po

src/backoffice/templates/backoffice/assembly_edit.html

src/backoffice/templates/backoffice/assembly_editlinks.html

src/backoffice/views/assemblies/assemblies.py