Buffered reader documentation for timeout 0

Some guys asked me what would happen if one is using the `BufferedReader.get_message` function with a timeout of 0. I checked the implementation and it looks like as this library relys on the Python `SimpleQueue` this is depending on the specific system. See here:
https://bugs.python.org/issue28982

I would like to suggest some changes at least to the documentation. I have the following proposals while not all of them might be done:
- Add information to the docstring that the timeout should always be a positive number (and maybe at least 10ms)
- Add a check for the timeout to be at least 10ms (or even 1ms)
- Add some information to the docstring that this relys on SimpleQueue and handling of a timeout of 0 or less is not specified

Please tell me your thoughts