bpo-28450: Fix and improve the documentation for unknown escapes in RE. (pythonGH-11920)

Author: serhiy-storchaka

Doc/library/re.rst

@@ -573,7 +573,8 @@ accepted by the regular expression parser::
 only inside character classes.)
 
 ``'\u'``, ``'\U'``, and ``'\N'`` escape sequences are only recognized in Unicode
-patterns.  In bytes patterns they are errors.
+patterns.  In bytes patterns they are errors.  Unknown escapes of ASCII
+letters are reserved for future use and treated as errors.
 
 Octal escapes are included in a limited form.  If the first digit is a 0, or if
 there are three octal digits, it is considered an octal escape. Otherwise, it is
@@ -850,7 +851,9 @@ form.
    *string* is returned unchanged.  *repl* can be a string or a function; if it is
    a string, any backslash escapes in it are processed.  That is, ``\n`` is
    converted to a single newline character, ``\r`` is converted to a carriage return, and
-   so forth.  Unknown escapes such as ``\&`` are left alone.  Backreferences, such
+   so forth.  Unknown escapes of ASCII letters are reserved for future use and
+   treated as errors.  Other unknown escapes such as ``\&`` are left alone.
+   Backreferences, such
    as ``\6``, are replaced with the substring matched by group 6 in the pattern.
    For example::