``has_distortion`` needs to be clearly documented

@astrofrog @mdboom 

`has_distortion` property is not documented. Based on the log entry for issue #2885 : _"... as well as a ``has_distortion`` property on the ``WCS`` class that indicates whether any distortions are present."_ It must be clarified what does _"any distortions are present"_ means. Based on the code, I understand that this is a convenience method that replaces multiple checks of ``sip is None``, ``cpdis1 is None``, etc. with a single call to `has_distortion`. However, ``has_distortion==False`` does not mean that the image is not distorted. Therefore, I believe, this must be made clear in the documentation.