Clarify Implications of Cargo Yank

[tustvold]

What does this PR try to resolve?

I found the documentation for cargo yank was not especially clear on the implications of yanking a crate, and I have seen this causing confusion within the community - https://github.com/tafia/quick-xml/issues/475.

On a somewhat related note, I have been observing lots more crates getting yanked recently and this is resulting in a fair amount of dependency upgrade busywork. I think/hope part of this is a documentation issue.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @ehuss (or someone else) soon.

Please see the contribution instructions for more information.

[Mingun]

I think it would be useful to add an example here

[adamreichold]

If the implications are discussed, I think mentioning the usual course of action (publishing a semver-compatible release before yanking) would be nice as well.

[ehuss]

r? @weihanglo