Backport: #1889 - fixup \xhh specs.

Doc/ref/ref2.tex

@@ -485,7 +485,7 @@ Notes:
 \item[(3)]
   As in Standard C, up to three octal digits are accepted.
 \item[(4)]
-  Unlike in Standard C, at most two hex digits are accepted.
+  Unlike in Standard C, exactly two hex digits are required.
 \item[(5)]
   In a string literal, hexadecimal and octal escapes denote the
   byte with the given value; it is not necessary that the byte