xo-unit: refactor: ++ docs

docs/basis-unit-constants.rst

@@ -0,0 +1,34 @@
+.. _basis-unit-constants:
+
+Basis Unit Constants
+====================
+
+These constants represent low-level building blocks.
+Relative scalefactors for each unit are chosen here.
+
+Application code will not use this class directly;
+instead it's expected to use units from the :code:`xo::qty::u` namespace.
+Those units are implemented on top of the basis units described here.
+
+.. code-block:: cpp
+
+    #include <xo/unit/basis_unit.hpp>
+
+    using xo::qty::detail::bu;
+
+Mass Units
+----------
+
+.. doxygenfunction:: xo::qty::abbrev::mass_unit2_abbrev
+
+.. doxygengroup:: basis-unit-mass-units
+
+Distance Units
+--------------
+
+.. doxygengroup:: basis-unit-distance-units
+
+Time Units
+----------
+
+.. doxygengroup:: basis-unit-time-units