Missing for-each Mut<T> indication for for-each Systems

[carrascomj]

Hello!

I started playing around with bevy (super cool) and it was quite exasperating to be unable to make for-each system with mutable components. Most of the frustration comes from the compiler giving useless feedback about not being able to build a system from the function. Also, most of the examples used query systems for these cases, so it was hard to find.

After converting everything to a query, I stumbled upon this in the Bevy-Cheatsheet:

You must use Mut instead of &mut T.

It would be cool if you could mention it in the book. I tried to look for it but I could not find it or, maybe, this pattern is discouraged, but it would also be worth mentioning it.

[cart]

Yeah the book is pretty lacking in a number of places, but Mut<T> is a pretty major omission that a lot of people slip up on. I'm delaying Book work while we stabilize things, but slipping Mut<T> in there now seems like the right call.

[carrascomj]

We can agree that this can be safely closed :) (bevyengine/bevy#798)