Async fn in trait information outdated

[Nereuxofficial]

Currently async in traits section claims that async fn cannot be used in traits in stable Rust. As of the release of Rust 1.75 this is no longer true.

The section should be updated to reflect the current situation of using async in traits, what limitations apply and why async_trait might still be used.

#215 adds a reference to the async fn in Traits announcement post, however does not yet clarify that this is as of now available in stable Rust and it's usage.