docs: flesh out bookkeeping and peer messaging

- numerous verbiage changes for clarity
- explain necessary guarantees for double-entry bookkeeping
- more explicit spec for use with ILP-over-HTTP
- rename transport/acc sys apis to "settlement engine callback api"
- rename API endpoints to match consensus/implementations
- remove confusing examples (more relevant for a tutorial)
- remove old idempotency section, new version is WIP