Add disk usage sections to cli pages.

This PR adds a section to the current `bin/omero fs` section to cover `usage`. This is very much a first draft. Suggestions of better handling of the `obj` arguments (which I now realise I have not specified other than in examples) and rephrasing the clunkier paragraphs would be most welcome.


Ah! I was blinded by the `bytes` and didn't notice the `MiB`. Sorry. This line is fine then.