docs: Add note on plan for service side PoW.

@Diziet plan for implementing service-side PoW as per our IRC discussion :)

The main difference between this and !2657 / what we've previously discussed is that this proposes using a Arc<Mutex<PowState>> that both the publisher and IptManager have as the method of communication for pow-params, rather than putting it in IptSet. This is to avoid TOCTOU races regarding which TPs we are publishing HsDescs for.