Give a heads up to users that turn a post into password-protected, to remove any SEO-related meta that they might not want

[leonidasmi]

A user could forget to wipe the metabox entries when they change a page to be password protected. And we can maybe warn them about that if we detect that.

If a user goes and fills the SEO meta for a post, it makes sense if we allowed them to do it. But there might be a chance that when they turn a post into password-protected, that they would prefer to wipe out that data.

We can be the good fellows here and give them a small heads up, before they do so.