add use case for juju-no-proxy to docs

[kwmonroe]

We mention no-proxy kinda as an aside:

For some services, e.g. apt, you may wish to also configure the no-proxy setting

There are occasions when juju-no-proxy is very important, e.g. LP2002646. We should expand on this use case better in our proxy docs. Specifically call out the scenarios where intra-cluster traffic needs to be direct to the apiserver/load-balancer (not proxied).