Update reference/constraints/DateTime.rst

Corrected class assertion for the PHP basic usage example.
[#2252] Backporting some changes from 2.1 thanks to @johnkary
[#1039] Adding a reference section for the global twig variables
[#1039] Tweaks thanks to @stof and @WouterJ
Merge branch '2.0' into 2.1

Conflicts:
	reference/forms/twig_reference.rst
[#1039][#2251] Updating the Form variables section with the new 2.1 variable
Merge remote-tracking branch 'origin/2.1' into 2.1