bevy_ecs: Add doc example for par_iter_mut (#11311) (#11499)

# Objective

Fixes #11311 

## Solution

Adds an example to the documentation for `par_iter_mut`. I didn't add
any examples to `par_iter`, because I couldn't think of a good example
and I figure users can infer that `par_iter` and `par_iter_mut` are
similar.

crates/bevy_ecs/src/query/state.rs

@@ -1150,6 +1150,9 @@ impl<D: QueryData, F: QueryFilter> QueryState<D, F> {
     ///
     /// This can only be called for read-only queries, see [`par_iter_mut`] for write-queries.
     ///
+    /// Note that you must use the `for_each` method to iterate over the
+    /// results, see [`par_iter_mut`] for an example.
+    ///
     /// [`par_iter_mut`]: Self::par_iter_mut
     #[inline]
     pub fn par_iter<'w, 's>(
@@ -1170,7 +1173,46 @@ impl<D: QueryData, F: QueryFilter> QueryState<D, F> {
     ///
     /// This can only be called for mutable queries, see [`par_iter`] for read-only-queries.
     ///
+    /// # Examples
+    ///
+    /// ```
+    /// use bevy_ecs::prelude::*;
+    /// use bevy_ecs::query::QueryEntityError;
+    ///
+    /// #[derive(Component, PartialEq, Debug)]
+    /// struct A(usize);
+    ///
+    /// # bevy_tasks::ComputeTaskPool::get_or_init(|| bevy_tasks::TaskPool::new());
+    ///
+    /// let mut world = World::new();
+    ///
+    /// # let entities: Vec<Entity> = (0..3).map(|i| world.spawn(A(i)).id()).collect();
+    /// # let entities: [Entity; 3] = entities.try_into().unwrap();
+    ///
+    /// let mut query_state = world.query::<&mut A>();
+    ///
+    /// query_state.par_iter_mut(&mut world).for_each(|mut a| {
+    ///     a.0 += 5;
+    /// });
+    ///
+    /// # let component_values = query_state.get_many(&world, entities).unwrap();
+    ///
+    /// # assert_eq!(component_values, [&A(5), &A(6), &A(7)]);
+    ///
+    /// # let wrong_entity = Entity::from_raw(57);
+    /// # let invalid_entity = world.spawn_empty().id();
+    ///
+    /// # assert_eq!(query_state.get_many_mut(&mut world, [wrong_entity]).unwrap_err(), QueryEntityError::NoSuchEntity(wrong_entity));
+    /// # assert_eq!(query_state.get_many_mut(&mut world, [invalid_entity]).unwrap_err(), QueryEntityError::QueryDoesNotMatch(invalid_entity));
+    /// # assert_eq!(query_state.get_many_mut(&mut world, [entities[0], entities[0]]).unwrap_err(), QueryEntityError::AliasedMutability(entities[0]));
+    /// ```
+    ///
+    /// # Panics
+    /// The [`ComputeTaskPool`] is not initialized. If using this from a query that is being
+    /// initialized and run from the ECS scheduler, this should never panic.
+    ///
     /// [`par_iter`]: Self::par_iter
+    /// [`ComputeTaskPool`]: bevy_tasks::ComputeTaskPool
     #[inline]
     pub fn par_iter_mut<'w, 's>(&'s mut self, world: &'w mut World) -> QueryParIter<'w, 's, D, F> {
         self.update_archetypes(world);

crates/bevy_ecs/src/system/query.rs

@@ -797,6 +797,9 @@ impl<'w, 's, D: QueryData, F: QueryFilter> Query<'w, 's, D, F> {
     ///
     /// This can only be called for read-only queries, see [`par_iter_mut`] for write-queries.
     ///
+    /// Note that you must use the `for_each` method to iterate over the
+    /// results, see [`par_iter_mut`] for an example.
+    ///
     /// [`par_iter_mut`]: Self::par_iter_mut
     /// [`World`]: crate::world::World
     #[inline]
@@ -814,6 +817,24 @@ impl<'w, 's, D: QueryData, F: QueryFilter> Query<'w, 's, D, F> {
     ///
     /// This can only be called for mutable queries, see [`par_iter`] for read-only-queries.
     ///
+    /// # Example
+    ///
+    /// Here, the `gravity_system` updates the `Velocity` component of every entity that contains it:
+    ///
+    /// ```
+    /// # use bevy_ecs::prelude::*;
+    /// #
+    /// # #[derive(Component)]
+    /// # struct Velocity { x: f32, y: f32, z: f32 }
+    /// fn gravity_system(mut query: Query<&mut Velocity>) {
+    ///     const DELTA: f32 = 1.0 / 60.0;
+    ///     query.par_iter_mut().for_each(|mut velocity| {
+    ///         velocity.y -= 9.8 * DELTA;
+    ///     });
+    /// }
+    /// # bevy_ecs::system::assert_is_system(gravity_system);
+    /// ```
+    ///
     /// [`par_iter`]: Self::par_iter
     /// [`World`]: crate::world::World
     #[inline]