We currently have two commands to build the documentation that do the same thing (or, will once #5156 is merged):

python setup.py build_docs
python setup.py build_sphinx

Right now AstropyBuildDocs subclasses AstropyBuildSphinx and is really just an alias. As discussed in astropy/astropy#5156, we could switch this so that AstropyBuildDocs is the primary command and add a deprecation warning to AstropyBuildSphinx.

This issue is paired with astropy/astropy#5157 - a changelog entry needs to be added to the main astropy repo for the deprecation, but the actual code needs to be changed here.