[compatibility] Add compatibility notices for pp-number (#713)

Author: tkoeppe

source/compatibility.tex

@@ -1511,6 +1511,21 @@
 string literal, as part of the implementation-defined mapping from physical
 source file characters to the basic source character set.
 
+\ref{lex.ppnumber}
+\change \grammarterm{pp-number} can contain \tcode{p} \grammarterm{sign} and
+\tcode{P} \grammarterm{sign}
+\rationale Necessary to enable hexadecimal floating literals.
+\effect
+Valid \CppXIV code may fail to compile or produce different results in
+this International Standard. Specifically, character sequences like \tcode{0p+0}
+and \tcode{0e1_p+0} are three separate tokens each in \CppXIV, but one single token
+in this International Standard.
+
+\begin{codeblock}
+#define F(a) b ## a
+int b0p = F(0p+0);  // ill-formed; equivalent to ``\tcode{int b0p = b0p + 0;}\!'' in C++ 2014
+\end{codeblock}
+
 \rSec2[diff.cpp14.expr]{Clause \ref{expr}: expressions}
 
 \ref{expr.post.incr}, \ref{expr.pre.incr}