Update `#[extendr]` usage documentation with breaking changes.

With the PR extendr/extendr#882, We should consider updating the documentation reflecting that in order for extendr functions to take custom rust structs as arguments, or return custom rustt structs, then they need to implement `T: TryFrom` and `Robj: From` respectively. which can be be donee now with annotating `#[extendr]` on those rust objects directly.

Also if the user wants to implement their own method, then they shouldn't use the `#[extendr]` macro on their structs.