"Methods from Deref<Target = [T]>" doesn't render const

[jplatte]

The documentation for [T]::is_empty on Vecs documentation page don't show that it's a const fn, while the documentation for the same method on [T]s documentation page correctly show that it is a const fn since 1.39.0.

@rustbot labels: +T-rustdoc +A-rustdoc-ui

[fmease]

This was intentionally introduced in #91291 as a fix for #90855 and it is going to be reverted once const_trait_impl is stable (#67792).