Closing issue1761.

Closing issue1761.
Surprising behaviour of the "$" regexp: it matches the
end of the string, AND just before the newline at the end
of the string::

    re.sub('$', '#', 'foo\n') == 'foo#\n#'

Python is consistent with Perl and the pcre library, so
we just document it.
Guido prefers "\Z" to match only the end of the string.