Strip unicode literal formatting from attribute documentation

On some doc pages for classes (and possibly modules)--for example [`Polynomial2D`](http://docs.astropy.org/en/v1.0.3/api/astropy.modeling.polynomial.Polynomial2D.html)--under the "Attribute Documentation" literal values that involve unicode strings display them with the Python 2 style `u'...'` unicode literal syntax. Since in the vast majority of cases users won't care about this, it's just distracting. Since we can't provide a different repr for every possible object that might contain a `str.__repr__` as part of its `__repr__`, a better approach, unfortunately, will be to just run a regexp over the final output when generating the attribute documentation.

Start by locating the code that generates class or module Attribute Documentation and inspect the final-output formatting path described in the issue. Reproduce the Polynomial2D documentation example and verify that unicode attribute values no longer display Python 2's u'...' prefix.

Written by the indexing model from the issue text.