[lex.pptoken] Reorder paragraphs to define terms before they are used (#7346)

First move p1 below p2, so that we do not refer to preprocessing tokens before they are defined.
Then move p4 up, as it is splitting some unrelated examples, neither of which use its contents.

Author: AlisdairM

source/lex.tex

@@ -509,11 +509,6 @@
     \textnormal{each non-whitespace character that cannot be one of the above}
 \end{bnf}
 
-\pnum
-Each preprocessing token that is converted to a token\iref{lex.token}
-shall have the lexical form of a keyword, an identifier, a literal,
-or an operator or punctuator.
-
 \pnum
 A preprocessing token is the minimal lexical element of the language in translation
 phases 3 through 6.
@@ -548,6 +543,22 @@
 between the quotation characters in a character literal or
 string literal.
 
+\pnum
+Each preprocessing token that is converted to a token\iref{lex.token}
+shall have the lexical form of a keyword, an identifier, a literal,
+or an operator or punctuator.
+
+\pnum
+The \grammarterm{import-keyword} is produced
+by processing an \keyword{import} directive\iref{cpp.import},
+the \grammarterm{module-keyword} is produced
+by preprocessing a \keyword{module} directive\iref{cpp.module}, and
+the \grammarterm{export-keyword} is produced
+by preprocessing either of the previous two directives.
+\begin{note}
+None has any observable spelling.
+\end{note}
+
 \pnum
 If the input stream has been parsed into preprocessing tokens up to a
 given character:
@@ -587,24 +598,14 @@
 \end{itemize}
 \end{itemize}
 
+\pnum
 \begin{example}
 \begin{codeblock}
 #define R "x"
 const char* s = R"y";           // ill-formed raw string, not \tcode{"x" "y"}
 \end{codeblock}
 \end{example}
 
-\pnum
-The \grammarterm{import-keyword} is produced
-by preprocessing an \keyword{import} directive\iref{cpp.import},
-the \grammarterm{module-keyword} is produced
-by preprocessing a \keyword{module} directive\iref{cpp.module}, and
-the \grammarterm{export-keyword} is produced
-by preprocessing either of the previous two directives.
-\begin{note}
-None has any observable spelling.
-\end{note}
-
 \pnum
 \begin{example}
 The program fragment \tcode{0xe+foo} is parsed as a