[lex.header] Modernize text around header names

The footnote better belongs in the main text as a regular note.
To make the notes flow consistently, switch the order of the
note and normative text in the first paragraph to lead with the
normative text.  Finally, take this opportunity to clarify the
specific characters called out using the Unicode nomenclature
used throughout this clause.

Author: AlisdairM

source/lex.tex

@@ -660,31 +660,32 @@
 \end{bnf}
 
 \pnum
+The sequences in both forms of \grammarterm{header-name}{s} are mapped in an
+\impldef{mapping header name to header or external source file} manner to headers or to
+external source file names as specified in~\ref{cpp.include}.
 \begin{note}
-Header name preprocessing tokens only appear within
+Header name preprocessing tokens appear only within
 a \tcode{\#include} preprocessing directive,
 a \tcode{__has_include} preprocessing expression, or
 after certain occurrences of an \tcode{import} token
 (see~\ref{lex.pptoken}).
 \end{note}
-The sequences in both forms of \grammarterm{header-name}{s} are mapped in an
-\impldef{mapping header name to header or external source file} manner to headers or to
-external source file names as specified in~\ref{cpp.include}.
 
 \pnum
-The appearance of either of the characters \tcode{'} or \tcode{\textbackslash} or of
+The appearance of either of the characters \unicode{0027}{apostrophe},
+\unicode{005c}{reverse solidus}, or of
 either of the character sequences \tcode{/*} or \tcode{//} in a
 \grammarterm{q-char-sequence} or an \grammarterm{h-char-sequence}
 is conditionally-supported with \impldef{meaning of \tcode{'}, \tcode{\textbackslash},
 \tcode{/*}, or \tcode{//} in a \grammarterm{q-char-sequence} or an
 \grammarterm{h-char-sequence}} semantics, as is the appearance of the character
-\tcode{"} in an \grammarterm{h-char-sequence}.
-\begin{footnote}
+\unicode{0022}{quotation mark} in an \grammarterm{h-char-sequence}.
+\begin{note}
 Thus, a sequence of characters
 that resembles an escape sequence can result in an error, be interpreted as the
 character corresponding to the escape sequence, or have a completely different meaning,
 depending on the implementation.
-\end{footnote}
+\end{note}
 \indextext{header!name|)}
 
 \rSec1[lex.ppnumber]{Preprocessing numbers}