Explain the purpose of `AppSystems` in `docs/patterns.md`

[benfrankel]

Why is it useful, why do everything in the Update schedule, and why those variants specifically?

[benfrankel]

As a starting point:

my opinion on AppSystems is that it's good practice to always put your systems in Update unless you know for sure that you can't, like if you have to run after a Bevy system in PostUpdate to avoid a 1-frame delay

that way you can adjust your system ordering more flexibly (because schedules are a hard boundary)

also that having a high-level system set for game logic just makes things much easier to reason about, and helps avoid 1-frame delays and difficult system ordering bugs, even if it technically introduces more ordering constraints than strictly necessary

[janhohenheim]

Note: this strategy introduces one frame delays the second you pull in Avian or start handling things in fixed updates yourself.
Not saying we need to fix that necessarily, but it could be worth documenting somewhere