[expr.const] Replace notes in sentences with footnotes.

Partially addresses #4228.

Author: Dawn Perchik

source/expressions.tex

@@ -6891,11 +6891,8 @@
 
 \item
 an invocation of a non-constexpr function
-\begin{note}
-Overload resolution\iref{over.match}
-is applied as usual.
-\end{note}%
-;
+\footnote{Overload resolution\iref{over.match}
+is applied as usual.};
 
 \item
 an invocation of an undefined constexpr function;
@@ -6920,13 +6917,10 @@
 \item
 an operation that would have undefined behavior
 as specified in \ref{intro} through \ref{cpp}
-\begin{note}
-including,
+\footnote{This includes,
 for example, signed integer overflow\iref{expr.prop}, certain
 pointer arithmetic\iref{expr.add}, division by
-zero\iref{expr.mul}, or certain shift operations\iref{expr.shift}
-\end{note}%
-;
+zero\iref{expr.mul}, or certain shift operations\iref{expr.shift}.};
 
 \item
 an lvalue-to-rvalue conversion\iref{conv.lval} unless