[Form] Adapt Symfony 2.3 changes in introduction chapter

[xabbuh]

The Form Component introduced some new features like the handleRequest() method and support for buttons in forms which need to be reflected in the introduction chapter of the Form Component.

• bind -> handleRequest (use handleRequest() instead of bind() #2974)
• bind -> submit
• ability to configure the method and URL of the form
• ability to render form "start" and "end"
• ability to configure buttons

[weaverryan]

In fact several things need to change in newer versions of the form component documentation. Some of these things may not be relevant to the component chapter (some of the "rendering" things), but I've listed them here so we can check.

Version 2.3

• bind -> handleRequest
• bind -> submit
• ability to configure the method and URL of the form
• ability to render form "start" and "end"
• ability to configure buttons

[xabbuh]

Thank you, @weaverryan. I added the task list to the pull request description.