Add text based on review comment.

See dart-lang#3937 (comment).

specification/dartLangSpec.tex

@@ -13058,7 +13058,8 @@ \subsubsection{Binding Actuals to Formals}
 \commentary{%
 Since it is illegal for a type parameter to be a supertype of its own
 bound, it follows that for any type $T$, there exists exactly one type
-$T_0$ for which $T$ is $T_0$ bounded.%
+$T_0$ for which $T$ is $T_0$ bounded. $T_0$ can be regarded as a
+non-type-variable type which is a (minimal) supertype of $T$.%
 }
 
 \LMHash{}%