rustdoc: the required const feature gate is not shown

[RalfJung]

For regular unstable functions, rustdoc shows something like this

However, for const-unstable functions, all we get is

There's no indication of what the feature gate is. Worse, if the function is unstable and const-unstable, but under different feature gates, then there's not even the "const: unstable" that links to the issue -- there's no way to figure out what the const feature gate is other than looking at the source.

Cc @rust-lang/rustdoc

[RalfJung]

Turns out the "const: unstable" has a tooltip indicating the const feature gate. But that can't be used for copy-paste, and it is quite hard to notice.