ARCHBOM-1636: add note about task_prerun

[robrap]

Description:

The celery signal task_prerun is a potential alternative for the decorator. This is being noted for our future selves, which will also help clarify that it wasn't yet tried.

JIRA:

ARCHBOM-1636

Merge checklist:

• [X] All reviewers approved
• [X] CI build is green
• [X] ~Version bumped~
• [X] ~Changelog record added~
• [X] Documentation updated (not only docstrings)
• [X] Commits are squashed

Post merge:

• [X] ~Create a tag~
• [X] ~Check new version is pushed to PyPi after tag-triggered build is finished.~
• [X] Delete working branch (if not needed anymore)