Ocean/pc doc201

[OceanNuclear]

Linked Issues

Closes #3600

Description

Add return docs into bluemira/radiation_transport.py

Interface Changes

N/A

Checklist

I confirm that I have completed the following checks:

• Tests run locally and pass pytest tests --reactor
• Code quality checks run locally and pass pre-commit run --from-ref develop --to-ref HEAD
• Documentation built locally and checked sphinx-build -W documentation/source documentation/build

---

📚 Documentation preview 📚: https://bluemira--3601.org.readthedocs.build/en/3601/

[je-cook]

Just a few little ones on so far

[OceanNuclear]

One of the static method in advective_transport was identical to the function of the same name in flux_surfaces_maker, so I've removed the former definition, and instead imported it from the latter file.