Add missing docs to World::change_tick and World::read_change_tick (

bevyengine#6765)

# Objective

The methods `World::change_tick` and `World::read_change_tick` lack documentation and have confusingly similar behavior.

## Solution

Add documentation and clarify the distinction between the two functions.

crates/bevy_ecs/src/world/mod.rs

@@ -1396,11 +1396,19 @@ impl World {
         self.change_tick.fetch_add(1, Ordering::AcqRel)
     }
 
+    /// Reads the current change tick of this world.
+    ///
+    /// If you have exclusive (`&mut`) access to the world, consider using [`change_tick()`](Self::change_tick),
+    /// which is more efficient since it does not require atomic synchronization.
     #[inline]
     pub fn read_change_tick(&self) -> u32 {
         self.change_tick.load(Ordering::Acquire)
     }
 
+    /// Reads the current change tick of this world.
+    ///
+    /// This does the same thing as [`read_change_tick()`](Self::read_change_tick), only this method
+    /// is more efficient since it does not require atomic synchronization.
     #[inline]
     pub fn change_tick(&mut self) -> u32 {
         *self.change_tick.get_mut()