Document rechunking logic and clarify terminology

[jbusecke]

Splitting this out of the large #715 as a separate issue.

          FWIW I think it would be helpful to keep working on a schematic like the one above and label the following terms in there:

• Files
• Fragments
• Sub-Fragments
• Groups (of fragments)
• Target chunks

I might have missed/double counted some terms, but having those enumerated might help with future debugging/refactoring.

Originally posted by @jbusecke in https://github.com/pangeo-forge/pangeo-forge-recipes/issues/715#issuecomment-2021853596