[func.memfn] Use math mode for indices and non-C++ ellipses

Author: jwakely

source/utilities.tex

@@ -14421,8 +14421,8 @@
 \begin{itemdescr}
 \pnum
 \returns A simple call wrapper~(\ref{func.def}) \tcode{fn}
-such that the expression \tcode{fn(t, a2, ..., aN)} is equivalent
-to \tcode{\placeholdernc{INVOKE}(pm, t, a2, ..., aN)}~(\ref{func.require}).
+such that the expression \tcode{fn(t, a$_2$, $\dotsc$, a$_N$)} is equivalent
+to \tcode{\placeholdernc{INVOKE}(pm, t, a$_2$, $\dotsc$, a$_N$)}~(\ref{func.require}).
 \end{itemdescr}
 \indextext{function object!\idxcode{mem_fn}|)}