Parallelize doc build
closes #15591 

https://travis-ci.org/jreback/pandas/builds/208594369

a couple of minutes faster with -j 2.

fixes some deprecated use of ``pd.Term``.

Note there is another error in HDFStore build, though I can't repro it locally.
Ideally, I think we should make it possible to pass this to `python make.py html` to specify it (for local usage)