Improve 0.9 -> 0.10 scheduling migration advice

[alice-i-cecile]

Fixes most of #603.

[alice-i-cecile]

I've omitted the other smaller fixes, as a) they're unrelated and b) I'm not immediately sure how to solve them. Please feel free to open more PRs to fix those.

[rparrett]

Just in case we want to address the custom stages part, I think these are equivalent.

// Bevy 0.9
#[derive(Debug, Hash, PartialEq, Eq, Clone, StageLabel)]
pub struct AfterUpdate;

app.add_stage_after(CoreStage::Update, AfterUpdate, SystemStage::parallel());

// Bevy 0.10
#[derive(Debug, Hash, PartialEq, Eq, Clone, SystemSet)]
#[system_set(base)]
pub struct AfterUpdate;

app.configure_set(
    AppSet::AfterUpdate
        .after(CoreSet::UpdateFlush)
        .before(CoreSet::PostUpdate),
);

One thing I wish the migration guide addressed more explicitly are scenarios where the user was (ab)using various core bevy stages because they need a command flush between systems.

We are "mostly" just saying (via code snippets) that StartupStage::PostUpdate is now StartupSet::PostUpdate. But this situation is so common that I think it would help to spell out in code how to use apply_system_buffers in that situation.

edit: It feels like we are talking about apply_system_buffers enough and in the right way, but not actually showing users how to use it.

[Malax]

Awesome to see better docs! Thanks for working on these! FWIW: My biggest hurdle migrating was the difference between in_set and in_base_set. Adding a system to a set via in_set would still add it to a base set. That caused

thread 'main' panicked at '`"Update"` and `"Render"` have a `before`-`after` relationship (which may be transitive) but share systems.'

errors that I wasn't able to understand without resorting to the amazing folks on Discord. (Thread: https://discord.com/channels/691052431525675048/1083571454404214885/1083571454404214885)

Would be cool to see this addressed in the migration notes, but I am not sure how. Given that I don't understand the new scheduling fully, I'm not really qualified to make suggestions :)

[rparrett]

Labels could be removed for brevity from the custom stages section since they are covered just below, if that's a concern.

[alice-i-cecile]

@rparrett I've adapted and added your suggestion, thank you!

@Malax excellent edge case, but I'm unsure how to address it here. This is definitely one of the rougher edges in the whole design IMO.

[djeedai]

Excellent, this is miles better than before, thanks a lot @alice-i-cecile!

Ouch on the subtle difference between OnEnter/OnExit and OnUpdate, that's a big footgun :(

[Weibye]

These are good changes and now is in a state where I would be able to migrate my apps. Everything I struggled with has been answered.

[alice-i-cecile]

Thanks a ton for the reviews, merging now :)