Update return type of persistent collection

Author: GromNaN

UPGRADE-3.0.md

@@ -100,3 +100,7 @@ The default implementation of `Type::closureToPHP` has been modified to call
 `convertToPHPValue` on the type class. This implementation was provided by
 the `ClosureToPHP` trait, which is deprecated. If you have custom types
 that use the `ClosureToPHP` trait, remove the trait.
+
+## Persistent Collections
+
+The return type of `PersistentCollection::add()` has changed from `true` to `void`.

src/PersistentCollection/PersistentCollectionTrait.php

@@ -440,12 +440,10 @@ public function set(string|int $key, mixed $value): void
      *
      * @param mixed $element The element to add.
      * @phpstan-param T $element
-     *
-     * @return true The return value is kept for BC reasons, but will be void in doctrine/mongodb-odm 3.0.
      */
-    public function add($element): bool
+    public function add($element): void
     {
-        return $this->doAdd($element, false);
+        $this->doAdd($element, false);
     }
 
     public function isEmpty(): bool