Update get_message doc to avoid confusion

[aiven-sal]

As reported in #150 the current get_message behavior is confusing.
Some people would reasonably expect it to default to being blocking.
We should state this more explicitly in the documentation.

We should also take a look at the rest of the code to see if there are other places with similar issues.

[aiven-sal]

We should also consider adding a run time warning as suggested in #150 (comment)