Add documentation to UnsafeWorldCell::increment_change_tick (#8697)

# Objective

This function does not have documentation.

## Solution

Copy the docs from `World::increment_change_tick`.

crates/bevy_ecs/src/world/mod.rs

@@ -1472,7 +1472,7 @@ impl World {
         }
     }
 
-    /// Increments the world's current change tick, and returns the old value.
+    /// Increments the world's current change tick and returns the old value.
     #[inline]
     pub fn increment_change_tick(&self) -> Tick {
         let prev_tick = self.change_tick.fetch_add(1, Ordering::AcqRel);

crates/bevy_ecs/src/world/unsafe_world_cell.rs

@@ -253,6 +253,7 @@ impl<'w> UnsafeWorldCell<'w> {
         unsafe { self.world_metadata() }.last_change_tick()
     }
 
+    /// Increments the world's current change tick and returns the old value.
     #[inline]
     pub fn increment_change_tick(self) -> Tick {
         // SAFETY: