Tbh, I downvoted you not because I don’t value quality communications, but because negotiating around your capacity is a small slice of professional communication, and your approach is needlessly adversarial. You will experience a lot of useless friction in your life by taking that approach.
Good communication isn’t adversarial: by the time it becomes adversarial, you’ve already lost. It can include solid documentation, taking the time to mentor others, respectful but clear code reviews, helping others argue your case for rescoping, presenting your work at meet-ups or conferences, hallway testing a new feature, listening to teammates explain an approach…
…and yes, sometimes negotiating capacity. If that feels like an adversarial conversation, then your manager sucks, and you should find a new one.