2d ColliderBuilder Too obvious docs for some fields

[UgurcanAkkok]

/// A structure responsible for building a new collider.
#[derive(Clone)]
#[cfg_attr(feature = "serde-serialize", derive(Serialize, Deserialize))]
pub struct ColliderBuilder {
    /// The shape of the collider to be built.
    pub shape: Shape,
    /// The density of the collider to be built.
    pub density: f32,
    /// The friction coefficient of the collider to be built.
    pub friction: f32,
    /// The restitution coefficient of the collider to be built.
    pub restitution: f32,
    /// The position of this collider relative to the local frame of the rigid-body it is attached to.
    pub delta: Isometry<f32>,
    /// Is this collider a sensor?
    pub is_sensor: bool,
}

This is just a suggestion that "The __ of the collider to be built" kind of documentations could be changed to something that helps understandability. They are too much obvious. As a person that has little knowledge about physics, i do not understand what "restitution", "density" means. And as a newcomer to this library, I do not understand what a sensor is. So those docs could be changed to one sentence explanation of what they are. Or something else that would fit better. Thanks.

[sebcrozet]

That's right, these are too obvious. Documentation improvements will come progressively.

[besok]

I believe, the issue can be closed