[Form] Time type doc on placeholders is outdated for 2.6+

[kix]

As far as I can see in the Symfony source, the placeholder for TimeType needs to be an array with hour, minute and second keys, although the doc now simply includes a quite irrelevant description of ChoiceType placeholder usage from /reference/forms/types/options/placeholder.rst.inc

[xabbuh]

Good catch @kix. Looks like something we need to fix.