docs: describe template support of properties-of<T>

docs/annotating_code/type_syntax/utility_types.md

@@ -9,8 +9,8 @@ and their respective types as values. This can be useful if you need to convert
 
 ```php
 class A {
-  public string $foo;
-  public int $bar;
+  public string $foo = 'foo!';
+  public int $bar = 42;
 
   /**
    * @return properties-of<self>
@@ -38,3 +38,32 @@ properties with a certain visibility:
 - `public-properties-of<T>`
 - `protected-properties-of<T>`
 - `private-properties-of<T>`
+
+
+### Limited template support
+
+As there is no way to statically analyze if a method returns all properties of a generic param (e.g. via Reflection or
+serialization), you have to annotate it where you assume it.
+
+```php
+/**
+ * @param T $object
+ * @return properties-of<T>
+ */
+public function asArray($object): array {
+  /** @var properties-of<T> */
+  $array = json_decode(json_encode($object), true);
+  return $array;
+}
+
+
+class A {
+  public string $foo = 'foo!';
+  public int $bar = 42;
+}
+
+$a = new A();
+$aAsArray = asArray($a);
+$aAsArray['foo']; // valid
+$aAsArray['adams']; // error!
+```