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.


Maybe this is because I am not sure what exactly ``positional arguments`` are. I would understand it so that if I specify both
``--units ``
and
``--groups``
the ``groups`` will be ignored ???? 
Seems somehow wrong, but this is what I understand from the sentence.