[docs] Update encoding lookup docs wrt #27938 (issue 28393)

[angwerzx at 126.com]

http://bugs.python.org/review/28393/diff/18859/Doc/library/codecs.rst
File Doc/library/codecs.rst (right):

http://bugs.python.org/review/28393/diff/18859/Doc/library/codecs.rst#newcode974
Doc/library/codecs.rst:974: dashes. Using alternative aliases for these
encodings may result in slower
Lack iso8859-1. And since there is utf8, also lacking utf16 and utf32.

http://bugs.python.org/review/28393/