Add force_set to Value API

[hydra]

In my 'makerpnp' application which uses a Dynamic<Message>, where the Message enum does not implement PartialEq because some of the values it contains are do not implement PartialEq, I found it very un-ergonomic to use map_mut and have been using the force_set from this PR instead for a while now.

example code from my app:

let ok_button = "Ok".into_button()
    .on_click({
        let message = self.message.clone();
        move |_event| message.force_set(Message::SomeEvent(event_args))
    });

I looked at calling force_set replace, but that already exists for types that implement PartialEq, also in my use-case the caller does not need the old value like set.

This PR also fixes the rustdoc documentation on the set method which didn't seem correct.