[concept.convertibleto] Add clarifying comment

Author: CaseyCarter

source/concepts.tex

@@ -348,7 +348,9 @@
 \begin{itemdecl}
 template <class From, class To>
 concept ConvertibleTo = is_convertible_v<From, To> &&
-  requires(From (&f)()) { static_cast<To>(f()); };
+  requires(From (&f)()) {   // f refers to an arbitrary function such that
+    static_cast<To>(f());   // f() is an equality-preserving expression
+  };
 \end{itemdecl}
 
 \begin{itemdescr}