new_without_default_derive help is confusing

[wfraser]

The current help text implies it's only for trivial cases that can be solved by typing #[derive(Default)], but the lint really fires for any struct that has a pub fn new() -> Self, even ones that have logic inside them (which can't be replaced with a #[derive(Default)]).

Help text should indicate that it means either doing the derive, or implementing Default by hand.

[rail-rain]

This is essentially a duplicate of #3697, which is younger; but got a detailed comment. The proposed change is slightly different form that one though.

[wfraser]

This lint has been long since improved.