Render stability attributes in rustdoc

[huonw]

/// Does something badly.
#[deprecated="use `fixed`"]
pub fn broken() { .. }

/// Does something well.
#[stable]
pub fn fixed() { .. }

Function broken

fn broken()

Stability: Deprecated[coloured red] use fixed

Does something badly.

Function fixed

fn fixed()

Stability: Stable[coloured e.g. green]

Does something well.

(Part of #6875 and #8125, continuation of #8921.)

[huonw]

cc @cmr, @Seldaek.