Implementer's Guide: Messaging cleanups (#1664)

* Multiple fix ups

* Remove missed HrmpOpenChannelRequestCount

* Fix the invariant string for HrmpOpenChannelRequestCount

* Update roadmap/implementers-guide/src/types/candidate.md

Co-authored-by: Bernhard Schuster <bernhard@ahoi.io>

* Add small clarification

* Fix another small typo

* Add a clarification to InboundHrmpMessage.sent_at

Co-authored-by: Bernhard Schuster <bernhard@ahoi.io>