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.


Hopefully the revise documentation with an example of positional arguments first should help clarify this point. I have added a example using --groups.