Add documentation for getChanges (#10258)

* Add documentation for getChanges

The eloquent method getOriginal is already documented, and we should do the same with getChanges.

* Update eloquent.md

---------

Co-authored-by: Taylor Otwell <[email protected]>

Author: Tamas-hi

eloquent.md

@@ -872,13 +872,36 @@ $user = User::find(1);
 $user->name; // John
 $user->email; // [email protected]
 
-$user->name = "Jack";
+$user->name = 'Jack';
 $user->name; // Jack
 
 $user->getOriginal('name'); // John
 $user->getOriginal(); // Array of original attributes...
 ```
 
+The `getChanges` method returns an array containing the attributes that changed when the model was last saved:
+
+```php
+$user = User::find(1);
+
+$user->name; // John
+$user->email; // [email protected]
+
+$user->update([
+    'name' => 'Jack',
+    'email' => '[email protected]',
+]);
+
+$user->getChanges();
+
+/*
+    [
+        'name' => 'Jack',
+        'email' => '[email protected]',
+    ]
+*/
+```
+
 <a name="mass-assignment"></a>
 ### Mass Assignment