Improve documentation on Update vs FixedUpdate schedule dichotomy (#14600)

# Objective

When looking at documentation for the `Update` schedule, its not
entirely obvious that developers should actually be using the
`FixedUpdate` schedule for most of their game logic. We should directly
cross-link between the two, and give examples of which systems to put in
which schedules.

## Solution

Do just that.

crates/bevy_app/src/main_schedule.rs

@@ -94,8 +94,16 @@ pub struct FixedFirst;
 #[derive(ScheduleLabel, Clone, Debug, PartialEq, Eq, Hash)]
 pub struct FixedPreUpdate;
 
-/// The schedule that contains most gameplay logic.
+/// The schedule that contains most gameplay logic, which runs at a fixed rate rather than every render frame.
+/// For logic that should run once per render frame, use the [`Update`] schedule instead.
 ///
+/// Examples of systems that should run at a fixed rate include (but are not limited to):
+/// - Physics
+/// - AI
+/// - Networking
+/// - Game rules
+///
+/// See the [`Update`] schedule for examples of systems that *should not* use this schedule.
 /// See the [`FixedMain`] schedule for details on how fixed updates work.
 /// See the [`Main`] schedule for some details about how schedules are run.
 #[derive(ScheduleLabel, Clone, Debug, PartialEq, Eq, Hash)]
@@ -128,9 +136,15 @@ pub struct FixedLast;
 #[derive(ScheduleLabel, Clone, Debug, PartialEq, Eq, Hash)]
 pub struct FixedMain;
 
-/// The schedule that contains app logic. Ideally containing anything that must run once per
+/// The schedule that contains any app logic that must run once per render frame.
-/// render frame, such as UI.
+/// For most gameplay logic, consider using [`FixedUpdate`] instead.
 ///
+/// Examples of systems that should run once per render frame include (but are not limited to):
+/// - UI
+/// - Input handling
+/// - Audio control
+///
+/// See the [`FixedUpdate`] schedule for examples of systems that *should not* use this schedule.
 /// See the [`Main`] schedule for some details about how schedules are run.
 #[derive(ScheduleLabel, Clone, Debug, PartialEq, Eq, Hash)]
 pub struct Update;