Add ide-assist: generate_documentation_from_trait

[A4-Tacks]

Generate documents from items defined in the trait.

Implement #20407 (comment)

Example

trait Foo {
    /// some docs
    fn foo(&self);
}
impl Foo for () {
    fn $0foo(&self) {}
}

->

trait Foo {
    /// some docs
    fn foo(&self);
}
impl Foo for () {
    /// some docs
    fn foo(&self) {}
}

[Veykril]

This does not seem useful to me. Why would you need to duplicate the docs from the trait to the impl? And if you do you can go to def and copy