Standard library rustdoc: show since when a function is const

[RalfJung]

The standard library has #[rustc_const_stable] attributes for all stable const fn. These contain since information. It would be great if rustdoc could render that, indicating since when this function is const-stable.

[jyn514]

What would you expect this to look like? For 'this has existed since version x' it shows up as a version number next to [src]:

But if there's both 'existed since' and 'const since', that's now ambiguous.

[RalfJung]

Strawman proposal: something like 1.0.0 (const: 1.10.0) in the same spot (and same color)?