Documented overriden form options
Update performance.rst
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
Merge branch '2.0' into 2.1

Conflicts:
	reference/forms/types/birthday.rst
	reference/forms/types/choice.rst
	reference/forms/types/form.rst
[#2397] Adding more empty_data details from previous changes that were removed when merging into 2.1
[#2397] Removing the csrf type - no such type exists in 2.1, it's added via a form type extension as a hidden field
[#2397] Updating error_bubbling default to be a bit more precise for 2.1
Merge branch '2.1' into 2.2