Treat "Failure" and "Safety note" doc comments specially in generated docs

[lkuper]

As noted on rust-dev, it's easy for these comments to blend too easily into the surrounding documentation. Something like a "watch out for this!" icon could help.

[huonw]

Visiting for triage. /cc @cmr.

[cmr]

#8125

/cc @Seldaek

[cmr]

So I'm going to defer this for a while. If we decide to use reST, like I would love to, this would be easily handled by a directive.

cc @chris-morgan

[Gankro]

cc me

[steveklabnik]

We now have # Panics for this.

[steveklabnik]

Triage: no change here.

[bluss]

Using headings is very unsatisfactory. I prefer **Panics** if you blah.

[steveklabnik]

Triage: no changes.