Goal: Provide a cookbook for downstream OpenStack services: require multiprocessing.set_start_method("spawn", force=True) at program entry before importing oslo.service when threading backend is used.

Acceptance Criteria:

• Verify markdown guide under doc/source/migration/spawn_method.rst.

• Guide includes before/after code snippets and troubleshooting FAQ.

• At least two reference projects (e.g., Nova, Neutron) acknowledge and link to the guide in their docs.

Additional Details:
See https://pythonspeed.com/articles/python-multiprocessing/ and video explanation at https://youtu.be/RIc-Tut95YM?t=185.