Make deprecation reasons more prominent

[kmcallister]

Right now you have to hold your mouse over a tiny bar next to the function name to find out why it's been deprecated.

[sinistersnare]

+1 maybe just hovering over a deprecated method and its description?

[Gankra]

An adhoc solution to this we've been using in some places is to replace the documentation of the method with the deprecated message.

[sinistersnare]

Some methods do this already. example: http://doc.rust-lang.org/std/vec/struct.Vec.html#method.append

Lets do that.

[kmcallister]

Let's keep the actual docs behind a "click to expand" button, and have rustdoc render the deprecation message as the initially visible part of the docs.

[sfackler]

Dup of #15468?

[caipre]

I'll take this.

[kmcallister]

Nominating for 1.0-stable.

[pnkfelix]

closing as dupe of #15468