Update Twig extension documentation to use 1.12 changes

[weaverryan]

This will apply to 2.1+. As of Twig 1.12, creating a Twig extension is slightly different. For example, Twig_Function_Function is replaced by Twig_SimpleFunction.

Both the old and new version of the Twig docs are maintained, which is nice for comparison:

Old: http://twig.sensiolabs.org/doc/advanced_legacy.html#creating-an-extension
New: http://twig.sensiolabs.org/doc/advanced.html#creating-an-extension

[ricardclau]

http://symfony.com/doc/master/cookbook/templating/twig_extension.html should be updated with the new way to go, right?

[wouterj]

Yes, Ryan is talking about that document.

[ricardclau]

Ok, I can take this one if you are ok with it.

[wouterj]

of course! Do what you can! :)