API comment conventions

[Rendered](https://github.com/steveklabnik/rfcs/blob/api_comment_conventions/text/0000-api-comment-conventions.md)

I'm mostly trying to codify what we already have consensus on, rather than address every possible question. I can forsee this kind of RFC getting very bogged down in a number of different details, and we can always add more things in a future RFC.
Could you note that plain text should be marked as `plain`, as per #500?