docs: "Interface Introduction" chapter updated

docs/users_guide/framework_basics/interface_introduction.md

@@ -22,6 +22,9 @@ behind this is that:
     a user's written code, a new idiom in the library is to use the same identifier for
     a type and its instance.
 
+    Also, to prevent possible issues in compile-time logic, all of the library's entities must be
+    marked `final`. This prevents the users to derive own strong types from them, which would
+    prevent expression template simplification of equivalent entities.
 
 ## Strong types instead of aliases
 
@@ -234,6 +237,17 @@ the resulting expression template.
     | `power<A, 3>, per<A>` |    `power<A, 2>`     |
     | `A, per<power<A, 2>>` | `{identity}, per<A>` |
 
+    It is important to notice here that only the elements with exactly the same type are being
+    simplified. This means that, for example, `m/m` results in `one`, but `km/m` will not be
+    simplified. The resulting derived unit will preserve both symbols and their relative
+    magnitude. This allows us to properly print symbols of some units or constants that require
+    such behavior. For example, the Hubble constant is expressed in `km⋅s⁻¹⋅Mpc⁻¹`, where both
+    `km` and `Mpc` are units of _length_.
+
+    Also, to prevent possible issues in compile-time logic, all of the library's entities must be
+    marked `final`. This prevents the users to derive own strong types from them, which would
+    prevent expression template simplification of equivalent entities.
+
 4. **Repacking**
 
     In case an expression uses two results of other operations, the components of its arguments are repacked