Clarify pre for arrays

Closes #2556
Note I also changed another part of pre for typographic reasons.
Currently I see:
y(tUnknown node type: sup)
In case mathjax isn't active (seems to be the case locally)
tpre looks weird, so t^-^ seems like the safest solution.

chapters/operatorsandexpressions.tex

@@ -1158,10 +1158,12 @@ \subsection{Event-Related Operators with Function Syntax}\doublelabel{event-rela
 positive number.\\ \hline
 
 % pre
-\lstinline!pre(y)! & Returns the \emph{left limit} $y(t\textsuperscript{pre})$ of
+\lstinline!pre(y)! & Returns the \emph{left limit} $y(t^{-})$ of
 variable $y(t)$ at a time instant $t$. At an event instant,
-$y(t\textsuperscript{pre})$ is the value of y after the last event
-iteration at time instant $t$ (see comment below). The \lstinline!pre()! operator can
+$y(t^{-})$ is the value of y after the last event
+iteration at time instant $t$ (see comment below). 
+Any subscripts in the component expression $y$ must be parameter expressions.
+The \lstinline!pre()! operator can
 be applied if the following three conditions are fulfilled
 simultaneously: (a) variable $y$ is either a subtype of a simple type or
 is a record component, (b) $y$ is a discrete-time expression (c) the