Write real docs for SystemSet (#19538)

# Objective

`SystemSet`s are surprisingly rich and nuanced, but are extremely poorly
documented.

Fixes #19536.

## Solution

Explain the basic concept of system sets, how to create them, and give
some opinionated advice about their more advanced functionality.

## Follow-up

I'd like proper module level docs on system ordering that I can link to
here, but they don't exist. Punting to follow-up!

---------

Co-authored-by: theotherphil <phil.j.ellison@gmail.com>

crates/bevy_ecs/src/schedule/set.rs

@@ -60,7 +60,93 @@ define_label!(
 );
 
 define_label!(
-    /// Types that identify logical groups of systems.
+    /// System sets are tag-like labels that can be used to group systems together.
+    ///
+    /// This allows you to share configuration (like run conditions) across multiple systems,
+    /// and order systems or system sets relative to conceptual groups of systems.
+    /// To control the behavior of a system set as a whole, use [`Schedule::configure_sets`](crate::prelude::Schedule::configure_sets),
+    /// or the method of the same name on `App`.
+    ///
+    /// Systems can belong to any number of system sets, reflecting multiple roles or facets that they might have.
+    /// For example, you may want to annotate a system as "consumes input" and "applies forces",
+    /// and ensure that your systems are ordered correctly for both of those sets.
+    ///
+    /// System sets can belong to any number of other system sets,
+    /// allowing you to create nested hierarchies of system sets to group systems together.
+    /// Configuration applied to system sets will flow down to their members (including other system sets),
+    /// allowing you to set and modify the configuration in a single place.
+    ///
+    /// Systems sets are also useful for exposing a consistent public API for dependencies
+    /// to hook into across versions of your crate,
+    /// allowing them to add systems to a specific set, or order relative to that set,
+    /// without leaking implementation details of the exact systems involved.
+    ///
+    /// ## Defining new system sets
+    ///
+    /// To create a new system set, use the `#[derive(SystemSet)]` macro.
+    /// Unit structs are a good choice for one-off sets.
+    ///
+    /// ```rust
+    /// # use bevy_ecs::prelude::*;
+    ///
+    /// #[derive(SystemSet, Debug, Clone, PartialEq, Eq, Hash)]
+    /// struct PhysicsSystems;
+    /// ```
+    ///
+    /// When you want to define several related system sets,
+    /// consider creating an enum system set.
+    /// Each variant will be treated as a separate system set.
+    ///
+    /// ```rust
+    /// # use bevy_ecs::prelude::*;
+    ///
+    /// #[derive(SystemSet, Debug, Clone, PartialEq, Eq, Hash)]
+    /// enum CombatSystems {
+    ///    TargetSelection,
+    ///    DamageCalculation,
+    ///    Cleanup,
+    /// }
+    /// ```
+    ///
+    /// By convention, the listed order of the system set in the enum
+    /// corresponds to the order in which the systems are run.
+    /// Ordering must be explicitly added to ensure that this is the case,
+    /// but following this convention will help avoid confusion.
+    ///
+    /// ### Adding systems to system sets
+    ///
+    /// To add systems to a system set, call [`in_set`](crate::prelude::IntoScheduleConfigs::in_set) on the system function
+    /// while adding it to your app or schedule.
+    ///
+    /// Like usual, these methods can be chained with other configuration methods like [`before`](crate::prelude::IntoScheduleConfigs::before),
+    /// or repeated to add systems to multiple sets.
+    ///
+    /// ```rust
+    /// use bevy_ecs::prelude::*;
+    ///
+    /// #[derive(SystemSet, Debug, Clone, PartialEq, Eq, Hash)]
+    /// enum CombatSystems {
+    ///    TargetSelection,
+    ///    DamageCalculation,
+    ///    Cleanup,
+    /// }
+    ///
+    /// fn target_selection() {}
+    ///
+    /// fn enemy_damage_calculation() {}
+    ///
+    /// fn player_damage_calculation() {}
+    ///
+    /// let mut schedule = Schedule::default();
+    /// // Configuring the sets to run in order.
+    /// schedule.configure_sets((CombatSystems::TargetSelection, CombatSystems::DamageCalculation, CombatSystems::Cleanup).chain());
+    ///
+    /// // Adding a single system to a set.
+    /// schedule.add_systems(target_selection.in_set(CombatSystems::TargetSelection));
+    ///
+    /// // Adding multiple systems to a set.
+    /// schedule.add_systems((player_damage_calculation, enemy_damage_calculation).in_set(CombatSystems::DamageCalculation));
+    /// ```
     #[diagnostic::on_unimplemented(
         note = "consider annotating `{Self}` with `#[derive(SystemSet)]`"
     )]