Add explanation of 400 Bad Request errors

As part of #324222 (closed), provide an explanation in https://docs.gitlab.com/ee/api/notes.html about the cause of a 400 Bad Request error.

I'm not certain I've addressed everything I can address in the issue, but I'm hesitant to make more changes that spell out how to add a body field to a request, because most of the examples already have one in the example. Kai - I'd appreciate hearing your take on how far you want me to take this work.

Related to #324222 (closed)