[lex.charset] Fix various issues with the description of UCNs.

Clarify that \U sequences not beginning 00 are ill-formed. Clarify
handling of code points naming reserved or noncharacter code points.
Remove unnecessary circumlocution through "short identifiers" by
directly talking about code points. Use code point values directly
rather than using C++ 0x notation.

[lex.string] Fix description of what UCNs mean, and convert it to a
note.

source/lex.tex

@@ -207,18 +207,17 @@
     \terminal{\textbackslash U} hex-quad hex-quad
 \end{bnf}
 
-The character designated by the \grammarterm{universal-character-name} \tcode{\textbackslash
-U00NNNNNN} is that character
-that has \tcode{U+NNNNNN} as a code point short identifier;
-the character designated by the \grammarterm{universal-character-name}
-\tcode{\textbackslash uNNNN} is that character
-that has \tcode{U+NNNN} as a code point short identifier.
-If a \grammarterm{universal-character-name} does not correspond to
-a code point in ISO/IEC 10646 or
-if a \grammarterm{universal-character-name} corresponds to
-a surrogate code point,
-the program is ill-formed. Additionally, if
-a \grammarterm{universal-character-name} outside
+A \grammarterm{universal-character-name}
+designates the character in ISO/IEC 10646 (if any)
+whose code point is the hexadecimal number represented by
+the sequence of \grammarterm{hexadecimal-digit}s
+in the \grammarterm{universal-character-name}.
+The program is ill-formed if that number is not a code point
+or if it is a surrogate code point.
+Noncharacter code points and reserved code points
+are considered to designate separate characters distinct from
+any ISO/IEC 10646 character.
+If a \grammarterm{universal-character-name} outside
 the \grammarterm{c-char-sequence}, \grammarterm{s-char-sequence}, or
 \grammarterm{r-char-sequence} of
 a character or
@@ -228,10 +227,10 @@
 \grammarterm{r-char-sequence}\iref{lex.string} does not form a
 \grammarterm{universal-character-name}.}
 \begin{note}
-ISO/IEC 10646 code points are within the range 0x0-0x10FFFF (inclusive).
-A surrogate code point is a value in the range 0xD800-0xDFFF (inclusive).
+ISO/IEC 10646 code points are integers in the range $[0, \mathrm{10FFFF}]$ (hexadecimal).
+A surrogate code point is a value in the range $[\mathrm{D800}, \mathrm{DFFF}]$ (hexadecimal).
 A control character is a character whose code point is
-in either of the ranges 0x0-0x1F or 0x7F-0x9F (both inclusive).
+in either of the ranges $[0, \mathrm{1F}]$ or $[\mathrm{7F}, \mathrm{9F}]$ (hexadecimal).
 \end{note}
 
 \pnum
@@ -1144,7 +1143,7 @@
 provided that the code point value
 can be encoded as a single UTF-8 code unit.
 \begin{note}
-That is, provided the code point value is in the range 0x0-0x7F (inclusive).
+That is, provided the code point value is in the range $[0, \mathrm{7F}]$ (hexadecimal).
 \end{note}
 If the value is not representable with a single UTF-8 code unit,
 the program is ill-formed.
@@ -1163,7 +1162,7 @@
 provided that the code point value is
 representable with a single 16-bit code unit.
 \begin{note}
-That is, provided the code point value is in the range 0x0-0xFFFF (inclusive).
+That is, provided the code point value is in the range $[0, \mathrm{FFFF}]$ (hexadecimal).
 \end{note}
 If the value is not representable
 with a single 16-bit code unit, the program is ill-formed.
@@ -1685,9 +1684,13 @@
 character requiring a surrogate pair, plus one for the terminating
 \tcode{u'\textbackslash 0'}. \begin{note} The size of a \tcode{char16_t}
 string literal is the number of code units, not the number of
-characters. \end{note} Within \tcode{char32_t} and \tcode{char16_t}
-string literals, any \grammarterm{universal-character-name}{s} shall be within the range
-\tcode{0x0} to \tcode{0x10FFFF}. The size of a narrow string literal is
+characters. \end{note}
+\begin{note}
+Any \grammarterm{universal-character-name}{s} are required to
+correspond to a code point in the range
+$[0, \mathrm{D800})$ or $[\mathrm{E000}, \mathrm{10FFFF}]$ (hexadecimal)\iref{lex.charset}.
+\end{note}
+The size of a narrow string literal is
 the total number of escape sequences and other characters, plus at least
 one for the multibyte encoding of each \grammarterm{universal-character-name}, plus
 one for the terminating \tcode{'\textbackslash 0'}.