Auto-trait documentation section empty with manual auto-trait impls

[RalfJung]

With #47833 merged, I expected to see auto-trait implementations in the nightly docs in a separate section. However, in case there are manual impls, they are still shown in the "Trait Implementations" section. For example, https://doc.rust-lang.org/nightly/std/cell/struct.Cell.html has the impls for Send and !Sync in the "Trait Implementations" section, leaving the "Auto Trait Implementations" section entirely empty.

At the very least, this looks funny and probably the section header should not be shown if the section is empty. However, I actually think it'd make more sense to move manual impls of auto-traits into that section -- this way, the place to look for Send and Sync impls is always the same, instead of depending on the type like it does now.

[GuillaumeGomez]

I don't see this bug anymore so I'll close the issue. Don't hesitate to reopen it if you find it again.