Rustdoc does not render stabilized attributes for associated constants

For example, [`Ipv4Addr::UNSPECIFIED`](https://doc.rust-lang.org/nightly/std/net/struct.Ipv4Addr.html#associatedconstant.UNSPECIFIED). 

Unlike in [`other methods`](https://doc.rust-lang.org/nightly/std/net/struct.Ipv4Addr.html#method.is_loopback) where it renders the version field from `#[stable(since = "a.b.c")]` .

@GuillaumeGomez 

cc #44348
