Improve 'raw=' keyword documentation

[jypeter]

I've had to convert a cdms2 variable to a np.ma variable and the official way seems to use raw=1 (raw=True seems to work as well)

The best (and only?) documentation of this is in 2.10.9. Variable Methods(cont’d), but the documentation should be improved

• The optional keyword argument raw specifies whether the return object is a variable or a masked array.

  • return => returned

• By default, a transient variable is returned, having the axes and attributes corresponding to 2,3 the region read. If raw=1, an MV2 masked array is returned, equivalent to the transient variable without the axis and attribute information.

  • what is 2,3?
  • an MV2 masked array is returned => a numpy.ma array is returned

>>> type(model_data_a)
<class 'cdms2.tvariable.TransientVariable'>

>>> type(np.array(model_data_a))
<type 'numpy.ndarray'>

>>> type(model_data_a(raw=1))
<class 'numpy.ma.core.MaskedArray'>

>>> type(model_data_a(raw=True))
<class 'numpy.ma.core.MaskedArray'>

[jypeter]

This is loosely related to #270 (comment)