Document NetCDF loading chunk control

[trexfeathers]

Follow on from #5399?

• Add documentation (e.g. docstrings, user guide content) for work completed in Finish NetCDF loading chunk control code #5399. Target the same feature branch as that work
  • (COMPLETED: Chunk Control documentation #5597)

[ESadek-MO]

(WIP)

• Add user guide sections of Chunk Control: NetCDF IO Tech Paper?
• Fix docstring tests
• Double check all changes have good docstrings