Explanation of sibling refs

[lornajane]

It would be helpful to get a short explanation of which contexts allow which siblings to a $ref as this causes quite a lot of confusion. There's a good comment from @handrews here: OAI/OpenAPI-Specification#3526 (comment) which might give us a basis to start from.

[handrews]

This should be at least partially addressed by the table of reference behaviors in PR #80. I plan to expand on that in future PRs, but my first try got a little over-ambitious (and a little too rant-y) so this PR is relatively limited.