[2.6] Document html5 option

[wouterj]

Since 2.6, the Date, DateTime and Time form fields have an option html5 which defaults to true. This option allows you to disable rendering a single widget as a html5 date type. This way, you can use some custom JS datepicker.

The option was introduced in symfony/symfony#8291 and renamed in symfony/symfony#11994

[stof]

one of the 2 issue references is wrong, as they point to the same issue

[wouterj]

@stof thanks for noting, change it :)

[weaverryan]

Thanks for catching this - it's rare that something gets merged without an issue first!

[wouterj]

@weaverryan Unfortunately, it happend a lot this 2.6 release...