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) {}
}