[cpp.include] Promote footnote to note

This commit promotes a footnote to a note, following the
style of the equivalent paragraph introduced with the
`embed` wording.

Author: AlisdairM

source/preprocessor.tex

@@ -752,13 +752,13 @@
 If the directive resulting after all replacements does not match
 one of the two previous forms, the behavior is
 undefined.
-\begin{footnote}
-Note that adjacent \grammarterm{string-literal}s are not concatenated into
+\begin{note}
+Adjacent \grammarterm{string-literal}s are not concatenated into
 a single \grammarterm{string-literal}
 (see the translation phases in~\ref{lex.phases});
 thus, an expansion that results in two \grammarterm{string-literal}s is an
 invalid directive.
-\end{footnote}
+\end{note}
 The method by which a sequence of preprocessing tokens between a
 \tcode{<}
 and a