[Form] Make FormInterface::add docblock more explicit

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

When replacing a child form from within a form event, it's not obvious that you have to use the 'add' method. Though it's minor, this will save somebody a google search.

src/Symfony/Component/Form/FormInterface.php

@@ -39,7 +39,7 @@ interface FormInterface extends \ArrayAccess, \Traversable, \Countable
     public function getParent();
 
     /**
-     * Adds a child to the form.
+     * Adds or replaces a child to the form.
      *
      * @param FormInterface|string|integer $child   The FormInterface instance or the name of the child.
      * @param string|null                  $type    The child's type, if a name was passed.