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.


The help does say, "Units to use for disk usage for the total size..." ie not the table entries. For that reason I thought the example should include `--report` to actually show this.