Clarify Where PeerManager::process_events happens in docs

[TheBlueMatt]

Val mentioned at #1043 (comment) that the current docs dont really point out explicitly that users shouldnt need to call process_events directly unless they're not using lightning-net-tokio or one of the other clients provided in our language bindings. We should make sure we cover that.

[vss96]

@TheBlueMatt Is this already done? If not I can pick it up.

[TheBlueMatt]

No, feel free to work on this.