add an example to update!

[CarloLucibello]

The update! docstring ends with the warning "However, you should not rely on the old model being fully updated." which maybe is a little unclear and vague.

I updated a bit the docstring and added an example. We could explain in which situation the old model won't be updated (scalar fields in a mutable struct?) if a simple explanation is available.

[mcabbott]

I made a suggestion below with attempt at demonstrating what it's doing. With a model where some arrays are immutable, you can see that the mutated original is only partly updated, and hence garbage.

I think that working through the struct Layer example here doesn't help much with understanding the difference between update and update!. But such things should be done somewhere, #84.

[ToucheSir]

LGTM