Improve module docstrings in "scimath.units"

[rahulporuri]

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 #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.

[homosapien-lcy]

Just want to clarify, do you mean adding thing like (red minus codes)
for all the unit defining files (such as units.area, units.energy)?