Improve the documentation of `Finder::exclude()`

src/Symfony/Component/Finder/Finder.php

@@ -386,6 +386,10 @@ class Finder implements \IteratorAggregate, \Countable
     /**
      * Excludes directories.
      *
+     * Directories passed as argument must be relative to the ones defined with the `in()` method. For example:
+     *
+     *     $finder->in(__DIR__)->exclude('ruby');
+     *
      * @param string|array $dirs A directory path or an array of directories
      *
      * @return $this