xo-unit: kitchen-sink doc + build improvements + 1 bug fix

docs/quantity-class.rst

@@ -3,26 +3,70 @@
 Quantity
 ========
 
+.. ditaa::
+
+    +----------------+---------------+
+    |cYEL quantity   |   xquantity   |
+    +----------------+---------------+
+    |          scaled_unit           |
+    +--------------------------------+
+    |         natural_unit           |
+    +--------------------------------+
+    |               bpu              |
+    +----------------+               |
+    |    bu_store    |               |
+    +----------------+---------------+
+    |           basis_unit           |
+    +--------------------------------+
+    |            dimension           |
+    +--------------------------------+
+
 .. code-block:: cpp
 
     #include <xo/unit/quantity.hpp>
 
+.. uml::
+    :scale: 99%
+    :align: center
+
+    allowmixing
+
+    object qty1<<quantity>>
+    qty1 : scale = 1.23
+
+    rectangle constexpr #e0f0ff {
+
+    object unit<<natural_unit>>
+
+    qty1 o-- unit : s_unit (static constexpr)
+
+    }
+
+Arithmetic on :doc:`xo::qty::quantity<quantity-reference>` instances
+does *not* use ``xo::qty::quantity::s_scaled_unit`` at runtime;
+instead gets everything it needs at compile time.
+
+Class
+-----
+
 The primary data structure for interacting with xo-unit is the
 template class ``xo::qty::quantity``.
+A quantity is a compile-time wrapper around a single arithmetic value,
+with type taken from the ``Repr`` parameter in ``quantity<Unit, Repr>``.
 
 .. doxygenclass:: xo::qty::quantity
 
-Type Traits
------------
-
-.. doxygengroup:: quantity-type-traits
-
 Member Variables
 ----------------
 
 .. doxygengroup:: quantity-static-vars
 .. doxygengroup:: quantity-instance-vars
 
+Type Traits
+-----------
+
+.. doxygengroup:: quantity-type-traits
+
 Constructors
 ------------