document has_units

Documentation for has_units is missing (a blank section has been stubbed out for it).

Contents of this section should:
- describe how the function should appear to clients
- describe how the decorator alters the function arguments
- describe how the return value is altered before it is passed to the caller
- provide an example function which makes explicit all of the above by showing an equivalent function with all the wrapper code included.
- give example of how (or if?) one can return a tuple of unitted values from a unitted function