Auto merge of #115078 - camelid:tydef-to-alias, r=aDotInTheVoid,GuillaumeGomez

rustdoc: Rename typedef to type alias

This matches the name used by the [Rust Reference][1], which is also what
people usually call these items.

[1]: https://doc.rust-lang.org/reference/items/type-aliases.html

r? `@GuillaumeGomez`

Author: bors