Check Asana task #5

- better describe 'smoothing' and 'spline' in Line
- drop 'axtype', 'axtype', 'tag', 'ref' in Annotation
- better describe 'x/yanchor', 'x/yref', 'align', 'x' and 'y' in Annotation and Legend
- better describe 'separators' and 'hidesources' in Layout
- drop 'labeloffset'
- a few typos here and there