Improve documentation of design decisions regarding the async runtime

There are some design decisions and potential footguns we avoid in the implementation of the Python client.

As was pointed out in PR #6, the documentation of these could be improved.
That PR was merged to have the bugfixes online early, but the feedback w.r.t. the documentation still should be applied.

Start by reading PR #6 and the async-runtime implementation to identify the design decisions and potential footguns referenced in the issue. Update the project documentation to explain those points clearly, with completion shown by the documentation covering the feedback that remained after PR #6 was merged.

Written by the indexing model from the issue text.