Better explain what does `Transform::mul_transform` does

## How can Bevy's documentation be improved?

Currently [`Transform::mul_transform`](https://docs.rs/bevy/latest/bevy/prelude/struct.Transform.html#method.mul_transform) has documentation "Multiplies self with transform component by component, returning the resulting [Transform](https://docs.rs/bevy/latest/bevy/transform/components/struct.Transform.html)". This is misleading:

- You need to check what components are and what they do
- It's not actual implementation as multiplication of translation vectors doesn't make sense

If the code was on [`Mat4`](https://docs.rs/bevy/latest/bevy/math/f32/struct.Mat4.html) it could depend on preexisting knowledge of how matrices work in 3D graphics. However absent of it, it should be something more like: "Multiplication creates a new transformation which first applies `transform` transformation and than `self`. Resulting transformation is returned". It might be nice to rename it to something like `combine` and `mul_vec` to apply if bevy tries to be friendly and avoid exposing user to transformation matrices.