[expr.sub] Add missing cross-references (#7688)

Author: Endilll

source/expressions.tex

@@ -3486,12 +3486,12 @@
 a possibly empty, comma-separated list of \grammarterm{initializer-clause}s
 that constitute the arguments to the subscript operator.
 The \grammarterm{postfix-expression} and
-the initialization of the object parameter of
-any applicable subscript operator function is sequenced before
+the initialization of the object parameter\iref{dcl.fct} of
+any applicable subscript operator function\iref{over.sub} is sequenced before
 each expression in the \grammarterm{expression-list} and also before
-any default argument.
+any default argument\iref{dcl.fct.default}.
 The initialization of a non-object parameter of
-a subscript operator function \tcode{S}\iref{over.sub},
+a subscript operator function \tcode{S},
 including every associated value computation and side effect,
 is indeterminately sequenced with respect to that of
 any other non-object parameter of \tcode{S}.