Admonition Guidelines

[felix920506]

(This should really be a discussion but I don't think discussions are enabled on this repo)

I think we can all agree that the docs have quite a bit of admonitions and not all of them are appropriate. However, there are still valid usages of admonitions. I would like to see a set of guidelines on the use of admonitions on this repo, instead of the the guidelines simply being "at the discretion of reviewers".