rustdoc: Include supertraits on inlined traits

Previously, documentation for an inlined trait (i.e. a trait imported
and reexported from another crate) didn't display the trait's
supertraits.

Closes #14636
auto merge of #14640 : tomjakubowski/rust/fix-14636, r=huonw

Previously, documentation for an inlined trait (i.e. a trait imported
and reexported from another crate) didn't display the trait's supertraits.

Closes #14636