docs: "canonical unit" added to glossary and its documentation in code was updated

docs/appendix/glossary.md

@@ -333,6 +333,20 @@
       associated with a specific quantity ([quantity specification](#quantity_spec) and
       [unit](#unit)).
 
+[`canonical representation of a unit, canonical unit`](#canonical-unit){ #canonical-unit }
+
+:   - A canonical representation of a unit consists of:
+        - a reference unit being the result of extraction of all the intermediate
+          [derived units](#derived-unit),
+        - a magnitude being a product of all the prefixes and magnitudes of extracted scaled units.
+    - All units having the same canonical unit are deemed equal.
+    - All units having the same reference unit are convertible
+      (their magnitude may differ and is used during conversion).
+
+[`reference unit`](#reference-unit){ #reference-unit }
+
+:   See [canonical representation of a unit](#canonical-unit)
+
 [`absolute quantity point origin`, `absolute point origin`](#absolute-point-origin){ #absolute-point-origin }
 
 :   - An explicit point on an axis of values of a specific [quantity](#quantity) type that serves

src/core/include/mp-units/unit.h

@@ -262,16 +262,13 @@ struct is_one<struct one> : std::true_type {};
 /**
  * @brief A canonical representation of a unit
  *
- * A canonical representation of a unit consists of a `reference_unit` and its scaling
+ * A canonical representation of a unit consists of:
- * factor represented by the magnitude `mag`.
+ * - a reference unit being the result of extraction of all the intermediate derived units,
- *
+ * - a magnitude being a product of all the prefixes and magnitudes of extracted scaled units.
- * `reference_unit` is a unit (possibly derived one) that consists only named base units.
- * All of the intermediate derived units are extracted, prefixes and magnitudes of scaled
- * units are stripped from them and accounted in the `mag`.
  *
  * All units having the same canonical unit are deemed equal.
- * All units having the same `reference_unit` are convertible (their `mag` may differ
+ * All units having the same reference unit are convertible (their magnitude may differ and
- * and is the subject of conversion).
+ * is used during conversion).
  *
  * @tparam U a unit to use as a `reference_unit`
  * @tparam M a Magnitude representing an absolute scaling factor of this unit