rustdoc bug rendering constant section

[SergioBenitez]

This is a reopening of #40370, which is still not entirely fixed:

The type is ContentType without the AngleBracketed { lifetimes: [], types: [], bindings: [] }.

@GuillaumeGomez, @frewsxcv

[GuillaumeGomez]

This const thing will drive me crazy. :-/

Checking what's wrong.