Improve module docstrings in "scimath.units"

At the moment, https://docs.enthought.com/scimath/units/available_units.html contains documentation on the available units in the various modules of `scimath.units`. Along with this, it would be useful to have API documentation which contains the same information. The PR https://github.com/enthought/scimath/pull/116 removes some of the module docstrings in `scimath.units.*` - which we would like to add back, along with adding module docstrings for the rest of the subpackage.