Python 3 bytes/str keys documentation: improve example

Make the behavior a bit clearer by only using two keys, removing the
call to incr_multi, and using more descriptive key names. It's too
easy to miss the `b` string prefix when the key name is also `b`.

Additionally, remove the broken call to sorted():

    >>> sorted(['1', b'2'])
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: unorderable types: bytes() < str()