Documented html5 option

| Q   | A
| --- | ---
| Doc fix? | no
| New docs? | yes (https://github.com/symfony/symfony/pull/8291, https://github.com/symfony/symfony/pull/11994)
| Applies to | 2.6+
| Fixed tickets | https://github.com/symfony/symfony-docs/issues/4262
[...] of an HTML type.