Private networks: graduate to stable feature

After someone complained about how reliable it is to use this experimental feature I can only say that we should graduate it to stable.

The docs say the criteria is

* A) More people using it and reporting how it works
* B) Better docs

I think A is fulfilled. We recommend using IPFS private networks when the usecase arises and I'm not aware of any really bad issues for ipfs itself. As a libp2p feature it is even more used. With cluster it never gave me issues.

I can get some work done on B).

(post 0.5.0)