MultiDark/CosmoSim: Documentation

There are some `TODO` items in the multidark docs and docstrings that need to be filled in, e.g.

 - [ ] https://github.com/astropy/astroquery/pull/267/files#diff-88b7848f737f07b4ac6d1620d1e4c13fR28
 - [ ] https://github.com/astropy/astroquery/pull/267/files#diff-88b7848f737f07b4ac6d1620d1e4c13fR227
 - [ ] https://github.com/astropy/astroquery/pull/267/files#diff-88b7848f737f07b4ac6d1620d1e4c13fR284
 - [ ] https://github.com/astropy/astroquery/pull/267/files#diff-4e92f72979f2e60bc1317b8dfc39d412R135