Fix wording on DetectChanges::is_changed (#8550)

# Objective

- Closes #8472 

## Solution

- Fix wording on DetectChanges::is_changed

Author: nicopap

crates/bevy_ecs/src/change_detection.rs

@@ -46,7 +46,12 @@ pub trait DetectChanges {
     /// Returns `true` if this value was added after the system last ran.
     fn is_added(&self) -> bool;
 
-    /// Returns `true` if this value was added or mutably dereferenced after the system last ran.
+    /// Returns `true` if this value was added or mutably dereferenced
+    /// either since the last time the system ran or, if the system never ran,
+    /// since the beginning of the program.
+    ///
+    /// To check if the value was mutably dereferenced only,
+    /// use `this.is_changed() && !this.is_added()`.
     fn is_changed(&self) -> bool;
 
     /// Returns the change tick recording the time this data was most recently changed.