Document behaviour of .insert() and .with() when component already exists

[zesterer]

Currently, the docs specify the following:

It would be nice to know what the behaviour of the following is, or at least whether the behaviour is undefined or not.

world.create_entity()
    .with(Pos(0, 0))
    .with(Pos(1, 1))
    .build();

[Aceeri]

Consecutive with and insert overwrite the previous component: https://github.com/slide-rs/specs/blob/master/src/storage/mod.rs#L404

Would agree it might be nice to have defined documentation for that, but I think it is pretty standard to overwrite.

[zesterer]

Thanks for that. Just wanted to make sure this applied in all cases.