Merge pull request #2704 from jkczyz/2023-11-channelmanager-docs

`ChannelManager` documentation refresh

diff --cc lightning/src/ln/channelmanager.rs
Simple merge

diff --cc lightning/src/offers/offer.rs
Simple merge

diff --cc lightning/src/offers/refund.rs
Simple merge

diff --cc lightning/src/sign/mod.rs
Simple merge