Document the fact that '\U' and '\u' escapes are not treated specially in 3.0 (see issue 2541)

Doc/reference/lexical_analysis.rst

@@ -423,8 +423,9 @@ characters that otherwise have a special meaning, such as newline, backslash
 itself, or the quote character.
 
 String literals may optionally be prefixed with a letter ``'r'`` or ``'R'``;
-such strings are called :dfn:`raw strings` and use different rules for
-interpreting backslash escape sequences.
+such strings are called :dfn:`raw strings` and treat backslashes as literal
+characters.  As a result, ``'\U'`` and ``'\u'`` escapes in raw strings are not
+treated specially.
 
 Bytes literals are always prefixed with ``'b'`` or ``'B'``; they produce an
 instance of the :class:`bytes` type instead of the :class:`str` type.  They
@@ -520,15 +521,6 @@ is more easily recognized as broken.)  It is also important to note that the
 escape sequences only recognized in string literals fall into the category of
 unrecognized escapes for bytes literals.
 
-When an ``'r'`` or ``'R'`` prefix is used in a string literal, then the
-``\uXXXX`` and ``\UXXXXXXXX`` escape sequences are processed while *all other
-backslashes are left in the string*. For example, the string literal
-``r"\u0062\n"`` consists of three Unicode characters: 'LATIN SMALL LETTER B',
-'REVERSE SOLIDUS', and 'LATIN SMALL LETTER N'. Backslashes can be escaped with a
-preceding backslash; however, both remain in the string.  As a result,
-``\uXXXX`` escape sequences are only recognized when there is an odd number of
-backslashes.
-
 Even in a raw string, string quotes can be escaped with a backslash, but the
 backslash remains in the string; for example, ``r"\""`` is a valid string
 literal consisting of two characters: a backslash and a double quote; ``r"\"``