docs: add note about custom activation strats in hosted proxies (#2173)
docs: add note about custom activation strats in hosted proxies (#2173)

## What

This change adds a note to the custom activation strategies section of
the proxy docs, clarifying that the hosted proxy can not use custom
activation strategies.

## Why

We had a customer asking whether this was possible and realized it's
not stated explicitly anywhere.
Unleash/unleash,Unleash/unleash,Unleash/unleash,Unleash/unleash