Cleanup Twig Documentation/Code

After working on #1927584: Handle trans block as Twig extension, I noticed that there is a LOT of documentation and code cleanup needed on Twig related stuff.

We should also probably be utilizing https://drupal.org/coding-standards/docs#defgroup properly to assert Twig in the "Themable" group and probably perhaps even a separate group just for Twig (ie: functions, filters, tags, etc.)