Why does Option<&str>::cloned() not produce an Option<String>?

Question

I have an Option<&str> and I would like to end up with an Option<String>. For other types, Option::cloned would be an easy way to do that. The docs even say:

pub fn cloned(self) -> Option<T>

Maps an Option<&T> to an Option<T> by cloning the contents of the option.

However, trying this actually results in a compile error:

fn main() {
    let unowned_str_maybe: Option<&str> = Some("abc");
    let owned_str_maybe: Option<String> = unowned_str_maybe.cloned();  // <-- unsatisfied trait bounds
    println!("{:?}", owned_str_maybe);
}

/* Compile error is:

 error[E0599]: the method `cloned` exists for enum `Option<&str>`, but its trait bounds were not satisfied
   --> src/main.rs:5:61
    |
5   |       let owned_str_maybe: Option<String> = unowned_str_maybe.cloned();  // <-- unsatisfied trait bounds
    |                                                               ^^^^^^ method cannot be called on `Option<&str>` due to unsatisfied trait bounds
    |
    = note: the following trait bounds were not satisfied:
            `str: Sized`
            `str: Clone`
            `Option<&str>: Iterator`
            which is required by `&mut Option<&str>: Iterator`
*/

Using a more verbose (less idiomatic?) construction of .map(|x| x.to_string()) does compile. Why does Option::cloned not work for converting &str to String?