documents changes to prefix replacement

Documentation changes related to conda/conda-build#195.

I attempted to build the docs, but got an error:

```
$ make html
sphinx-build -b html -d build/doctrees   source build/html
Making output directory...
Running Sphinx v1.2.2

Exception occurred:
  File "conf.py", line 110, in <module>
ImportError: No module named continuum_sphinx_theme
```