Improved the phpdoc of form actions on elements to describe the behavior

This enhances the description of form actions in the element API. I decided to use ``@see`` in the DriverInterface phpdoc to link to NodeElement rather than duplicating the documentation, to ease the maintenance. The link is done this way rather the opposite so that Mink users don't need to look at the DriverInterface to understand the public API.
This documentation expects us to fix #526 and #519 as it describe them as supported. I'm not adding any new test to enforce this in the current PR as extensive tests for the tricky cases of form actions are following in separate PRs.