Rephrase introduction to vectorization rules

The previous formulation had a passive form that didn't really work as an introduction to the list of rules.

chapters/functions.tex

@@ -822,10 +822,9 @@ \subsection{Scalar Functions Applied to Array Arguments}\label{scalar-functions-
 
 Functions with one scalar return value can be applied to arrays element-wise, e.g.\ if \lstinline!A! is a vector of reals, then \lstinline!sin(A)! is a vector where each element is the result of applying the function \lstinline!sin! to the corresponding element in \lstinline!A!.  Only \lstinline!function! classes that are transitively non-replaceable (\cref{transitively-non-replaceable} and \cref{restrictions-on-base-classes-and-constraining-types-to-be-transitively-non-replaceable}) may be called vectorized.
 
-Consider the expression \lstinline!f(arg1, $\ldots$, argn)!, an application of the function \lstinline!f! to the arguments \lstinline!arg1!, \ldots, \lstinline!argn! is defined.
-
-For each passed argument, the type of the argument is checked against
-the type of the corresponding formal parameter of the function.
+Consider the expression \lstinline!f(arg1, $\ldots$, argn)!, an application of the function \lstinline!f! to the arguments \lstinline!arg1!, \ldots, \lstinline!argn!.
+Potential vectorization of this call is defined as follows.
+For each passed argument, the type of the argument is checked against the type of the corresponding formal parameter of the function:
 \begin{enumerate}
 \item\label{argument-type-check-match}
   If the types match, nothing is done.