docs: Added Set::forEach() documentation

Author: rudashi

docs/Set.md

@@ -9,6 +9,7 @@ The Set object holds only unique values of any type.
 - [`Set::clear()`](#setclear)
 - [`Set::delete()`](#setdelete)
 - [`Set::entries()`](#setentries)
+- [`Set::forEach()`](#setforeach)
 - [`Set::has()`](#sethas)
 - [`Set::size`](#setsize)
 
@@ -100,6 +101,18 @@ $iterator->current();
 // ['foo', ''foo']
 ```
 
+### Set::forEach()
+
+The `forEach()` method executes a provided function once per each element of the Set.
+
+```php
+$set = new Set([3, 0]);
+
+$set->forEach(function (int $value1, int $value2, Set $set) {
+    // ...
+});
+```
+
 ### Set::has()
 
 The `has()` method returns `TRUE` when element with the specified value exists in the `Set` or `FALSE` if it does not exist.