A completely idle `transport_worker` thread is reported as `0.0%` idle,
which is confusing. Moreover the docs on the network threading model do
not reflect the changes made in #90482. This commit fixes both of those
things.
Currently the documentation on network threading suggests that we still
use a model where we have individual workers dedicated to server
sockets. That is no longer true and server sockets are assigned to
normal workers. This commit updates the documentation.
