Documented missing FormType variables
Documented checked variable
Documented choice variables
Documented selectedchoice test
Added performance tip
Documented CollectionType variables
Documented Date(Time)Type variables
Extracted duplicated var tables
Documented date_pattern variable of DateType
Documented FileType variables
Documented MoneyType variables
Documented SubmitType variables
Documented TimeType variables
Applied comments by @xabbuh
Made full stop usage consistent
feature #3588 Documented all form variables (WouterJ)

This PR was merged into the 2.3 branch.

Discussion
----------

Documented all form variables

| Q   | A
| --- | ---
| Doc fix? | no
| New docs? | yes
| Applies to | 2.3+
| Fixed tickets | #2372

This PR adds all missing global form variables to the twig reference.

Besides that, it also introduces a new section for form types: Form
Variables. This explains what kind of variables are available for that
particular field. This is usefull if you want to create a custom theme.

At last, it also adds the missing docs for the `selectedchoice` Twig
test.

Commits
-------

7710429 Made full stop usage consistent
398d23f Applied comments by @xabbuh
97d216d Documented TimeType variables
8b5ecf9 Documented SubmitType variables
688409e Documented MoneyType variables
a811833 Documented FileType variables
e5fded9 Documented date_pattern variable of DateType
1a1497c Extracted duplicated var tables
95540ca Documented Date(Time)Type variables
3bfdc20 Documented CollectionType variables
754da30 Added performance tip
7d10315 Documented selectedchoice test
c8a44ae Documented choice variables
98e828f Documented checked variable
d2b57be Documented missing FormType variables