docs: minor cleanup of quantity hierarchies

docs/users_guide/framework_basics/dimensionless_quantities.md

@@ -256,20 +256,20 @@ Angular quantities are not the only ones with such a "strange" behavior. Another
 is a _storage capacity_ quantity specified in IEC-80000-13 that again allows expressing it in both
 `one` and `bit` units.
 
-Those cases make dimensionless quantities an exceptional tree in the library. This is the only
-[quantity hierarchy](../../appendix/glossary.md#quantity-hierarchy) that contains more than one
-[quantity kind](../../appendix/glossary.md#kind) in its tree:
+Those cases make dimensionless quantities an exceptional tree in the library. This
+[quantity hierarchy](../../appendix/glossary.md#quantity-hierarchy) contains more than one
+[quantity kind](../../appendix/glossary.md#kind) and more than one unit in its tree:
 
 ```mermaid
 flowchart TD
     dimensionless["<b>dimensionless</b><br>[one]"]
     dimensionless --- rotation["<b>rotation</b>"]
-    dimensionless --- efficiency["<b>efficiency</b>"]
-    dimensionless --- angular_measure["<b>angular_measure</b><br>[rad]"]
-    angular_measure --- rotational_displacement["<b>rotational_displacement</b>"]
+    dimensionless --- thermodynamic_efficiency["<b>thermodynamic_efficiency</b><br><i>(work / heat)</i>"]
+    dimensionless --- angular_measure["<b>angular_measure</b><br><i>(arc_length / radius)</i><br>[rad]"]
+    angular_measure --- rotational_displacement["<b>rotational_displacement</b><br><i>(path_length / radius)</i>"]
     angular_measure --- phase_angle["<b>phase_angle</b>"]
-    dimensionless --- solid_angular_measure["<b>solid_angular_measure</b><br>[sr]"]
-    dimensionless --- drag_factor["<b>drag_factor</b>"]
+    dimensionless --- solid_angular_measure["<b>solid_angular_measure</b><br><i>(area / pow<2>(radius))</i><br>[sr]"]
+    dimensionless --- drag_factor["<b>drag_factor</b><br><i>(drag_force / (mass_density * pow<2>(speed) * area))</i>"]
     dimensionless --- storage_capacity["<b>storage_capacity</b><br>[bit]"] --- equivalent_binary_storage_capacity["<b>equivalent_binary_storage_capacity</b>"]
     dimensionless --- ...
 ```

docs/users_guide/framework_basics/systems_of_quantities.md

@@ -296,7 +296,7 @@ sometimes not obvious what such a tree should look like. Also, ISO explicitly st
 
     The division of ‘quantity’ according to ‘kind of quantity’ is, to some extent, arbitrary.
 
-The below presents some arbitrary hierarchy of derived quantities of kind energy:
+The below presents some arbitrary hierarchy of derived quantities of kind _energy_:
 
 ```mermaid
 flowchart TD
@@ -365,7 +365,7 @@ quantities of the same kind. Such quantities have not only the same dimension bu
 can be expressed in the same units.
 
 To annotate a quantity to represent its kind (and not just a hierarchy tree's root quantity)
-we introduced a `kind_of<>` specifier. For example, to express any quantity of length, we need
+we introduced a `kind_of<>` specifier. For example, to express any quantity of _length_, we need
 to type `kind_of<isq::length>`.
 
 !!! important