Sized trait should have #[doc(alias = "?")]

[Zalathar]

Summary

Typing ? into the standard library’s search field shows aliased links to the Try trait (and the old try! macro), but there is no link to the Sized trait, which frequently appears as ?Sized in generic type bounds.

This could be a way to lead some users towards the documentation for this unusual use of the question mark character in trait-bound syntax.

Inspired by this reddit comment, since it turns out that the current docs would not have helped that person find their answer.

[GuillaumeGomez]

I opened #102102 to fix this. I also added ?Sized as a doc alias.