Add section to book about Mut wrappers and the ergonomics

[alice-i-cecile]

So what you're running into is when you request a &mut Position in your system, Bevy hands you Mut<Position> instead.

Mut is a "smart pointer" used to implement change detection. When it's mutably dereferenced, Bevy says "hey, this value was probably changed!". The downside of this is that the ergonomics are sometimes surprising.

Mut implements Deref and DerefMut, so fields and methods are "forwarded" to the internal type. That's why setting the field values works. But to assign to the internal value (and not the smart pointer), you need to dereference it first. Same with doing things like adding or subtracting the values with straightforward trait impls

• by me

We should probably talk about matching as well.

I'm not sure what section of the book this belongs in, maybe a "frequently encountered problems" section? Maybe an aside in the section on queries?