Enhanced par_for_each and par_for_each_mut docs (#4039)

# Objective
Continuation of  #2663 due to git problems - better documentation for Query::par_for_each and par_for_each_mut

## Solution
Going into more detail about the function parameters

crates/bevy_ecs/src/system/query.rs

@@ -503,10 +503,26 @@ where
         };
     }
 
-    /// Runs `f` on each query result in parallel using the given task pool.
+    /// Runs `f` on each query result in parallel using the given [`TaskPool`].
     ///
     /// This can only be called for immutable data, see [`Self::par_for_each_mut`] for
     /// mutable access.
+    ///
+    /// # Tasks and batch size
+    ///
+    /// The items in the query get sorted into batches.
+    /// Internally, this function spawns a group of futures that each take on a `batch_size` sized section of the items (or less if the division is not perfect).
+    /// Then, the tasks in the [`TaskPool`] work through these futures.
+    ///
+    /// You can use this value to tune between maximum multithreading ability (many small batches) and minimum parallelization overhead (few big batches).
+    /// Rule of thumb: If the function body is (mostly) computationally expensive but there are not many items, a small batch size (=more batches) may help to even out the load.
+    /// If the body is computationally cheap and you have many items, a large batch size (=fewer batches) avoids spawning additional futures that don't help to even out the load.
+    ///
+    /// # Arguments
+    ///
+    ///* `task_pool` - The [`TaskPool`] to use
+    ///* `batch_size` - The number of batches to spawn
+    ///* `f` - The function to run on each item in the query
     #[inline]
     pub fn par_for_each<FN: Fn(<Q::ReadOnlyFetch as Fetch<'w, 's>>::Item) + Send + Sync + Clone>(
         &'s self,
@@ -529,7 +545,8 @@ where
         };
     }
 
-    /// Runs `f` on each query result in parallel using the given task pool.
+    /// Runs `f` on each query result in parallel using the given [`TaskPool`].
+    /// See [`Self::par_for_each`] for more details.
     #[inline]
     pub fn par_for_each_mut<'a, FN: Fn(<Q::Fetch as Fetch<'a, 'a>>::Item) + Send + Sync + Clone>(
         &'a mut self,