Add documentation for r/Rvir normalization feature of pairwise velocities

The [vrad tutorial](http://halotools.readthedocs.io/en/latest/quickstart_and_tutorials/tutorials/catalog_analysis/galcat_analysis/basic_examples/velocity_statistics/galaxy_catalog_analysis_tutorial6.html) and [vdisp tutorial](http://halotools.readthedocs.io/en/latest/quickstart_and_tutorials/tutorials/catalog_analysis/galcat_analysis/basic_examples/velocity_statistics/galaxy_catalog_analysis_tutorial7.html) still do not contain anything about this new feature, nor do the docstrings, so the `normalize_rbins_by` feature is still mostly a hidden feature.