[ExpressionLanguage] added some documentation about functions

src/Symfony/Component/ExpressionLanguage/ExpressionLanguage.php

@@ -92,6 +92,18 @@ public function parse($expression, $names)
     /**
      * Registers a function.
      *
+     * A function is defined by two PHP callables. The callables are used
+     * by the language to compile and/or evaluate the function.
+     *
+     * The first function is used at compilation time and must return a
+     * PHP representation of the function call (it receives the function
+     * arguments as arguments).
+     *
+     * The second function is used for expression evaluation and must return
+     * the value of the function call based on the values defined for the
+     * expression (it receives the values as a first argument and the function
+     * arguments as remaining arguments).
+     *
      * @param string   $name      The function name
      * @param callable $compiler  A callable able to compile the function
      * @param callable $evaluator A callable able to evaluate the function