Feature request: show rustc_const_unstable in generated documentation

[ChaiTRex]

Describe the problem you are trying to solve

I looked at the documentation for core::mem::variant_count. It does mention that the function is nightly-only:

🔬 This is a nightly-only experimental API.
(variant_count #73662)

It misleadingly doesn't mention that const fn can also be enabled on nightly, which requires opening the [src] link and noticing:

#[rustc_const_unstable(feature = "variant_count", issue = "73662")]

Describe the solution you'd like

When the rustc_const_unstable attribute is applied to a function, show that, much like nightly-only functions are shown in the documentation.

[ehuss]

Transferred to rust-lang/rust, as that is where rustdoc lives (which generates the documentation).

[jyn514]

It would be nice to show something like (const: unstable) to mirror how shows rustdoc shows stable annotations like (const: 1.50).

[steffahn]

Looks like in current beta (or nightly) it shows const for method (at least for pointer’s methods) unconditionally, without much or any indication that a function may only be stable without the const.

[jyn514]

Added in #86473