Added commentary

specification/dartLangSpec.tex

@@ -11762,7 +11762,12 @@ \subsection{Function Expressions}
 \commentary{%
 Note that if $T$ derives a future type $F$ then \SubtypeNE{T}{F},
 and $F$ is always of the form \code{$G$<...>} or \code{$G$<...>?},
-where $G$ is \code{Future} or \code{FutureOr}.%
+where $G$ is \code{Future} or \code{FutureOr}.
+Also note that 'derives' in this context refers to the computation
+where a type $T$ is given, the supertypes of $T$ are searched,
+and a type of one of those forms is selected.
+There is no connection to the notion of a 'derived class' meaning 'subclass'
+that some programming language communities use.%
 }
 
 \LMHash{}%