[docs] [issue28088] Document Transport.set_protocol and get_protocol

[INADA Naoki]

INADA Naoki added the comment:

lgtm.

But I think adding note like following may be helpful to avoid users try
switching protocols which protocol author doesn't expect.
(I'm not good English writer.  I hope someone polish my sentence).

.. note::

   Generally speaking, switching protocols requires special knowledge
   about two protocols. For example, old protocol may have inner receive
   buffer and new protocol should take over it.
   Transport doesn't take care about such issues. Protocols should support
   switching protocol using this API.

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nosy: +inada.naoki

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