[lex.pptoken] Turn non-normative text into a note

The last part of this paragraph is non-normative, so turn it
into a note.  Also, the preceding sentence defining whitespace
characters is mostly unrelated to the precedingd defintion of
preprocessing tokens, so start a new paragraph to more clearly
show the comment assoication.

Author: AlisdairM

source/lex.tex

@@ -563,6 +563,8 @@
 matches the last category, the program is ill-formed.
 If any character not in the basic character set matches the last category,
 the program is ill-formed.
+
+\pnum
 Preprocessing tokens can be separated by
 \indextext{whitespace}%
 whitespace;
@@ -573,12 +575,14 @@
 new-line,
 \unicode{000b}{line tabulation}, and
 \unicode{000c}{form feed}), or both.
+\begin{note}
 As described in \ref{cpp}, in certain
 circumstances during translation phase 4, whitespace (or the absence
 thereof) serves as more than preprocessing token separation. Whitespace
 can appear within a preprocessing token only as part of a header name or
 between the quotation characters in a character literal or
 string literal.
+\end{note}
 
 \pnum
 Each preprocessing token that is converted to a token\iref{lex.token}