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.


``This command takes optional positional arguments of object types and ids to and…`` - this is unclear - ``to …`` what ?