Documented overriden form options
Merge branch 'form_overriden_options_20' of github.com:WouterJ/symfony-docs into WouterJ-form_overriden_options_20

Conflicts:
	reference/forms/types/birthday.rst
	reference/forms/types/choice.rst
	reference/forms/types/date.rst
	reference/forms/types/time.rst
[#2397] Tweaks for new overridden form options