remove ParallelIterator

crates/bevy_ecs/src/system/query.rs

@@ -219,7 +219,14 @@ 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 task pool. This is
+    /// the parallel equivalent of [`Self::for_each`]
+    ///
+    /// Note that the overhead of ParallelIterator is high relative to some
+    /// workloads. In particular, if the batch size is too small or task being
+    /// run in parallel is inexpensive, *a `par_for_each` could take longer
+    /// than a normal [`Self::for_each`]*. Therefore, you should profile your code before
+    /// using ParallelIterator.
     ///
     /// This can only be called for read-only queries, see [`Self::par_for_each_mut`] for
     /// write-queries.
@@ -246,7 +253,10 @@ 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 task pool. This is
+    /// the parallel equivalent of [`Self::for_each_mut`]
+    ///
+    /// See [`Self::par_for_each`] for more information on performance.
     #[inline]
     pub fn par_for_each_mut(
         &mut self,