[expr] Use 'possibly converted' for discarded-value expression.

Fixes #1597.

Author: jensmaurer

source/expressions.tex

@@ -238,7 +238,7 @@
 \begin{note} Using an overloaded operator causes a function call; the
 above covers only operators with built-in meaning.
 \end{note}
-If the expression is a prvalue after this optional conversion,
+If the (possibly converted) expression is a prvalue,
 the temporary materialization conversion~(\ref{conv.rval}) is applied.
 \begin{note}
 If the expression is an lvalue of