Documented that we now support Twig 2.1+

Author: weierophinney

doc/book/features/template/twig.md

@@ -127,3 +127,8 @@ return [
     ],
 ];
 ```
+
+When specifying the `twig.extensions` values, always use fully qualified class
+names or actual extension instances to ensure compatibility with any version of
+Twig used. Version 2 of Twig _requires_ that a fully qualified class name is
+used, and not a short-name alias.

doc/book/reference/migration/to-v2.md

@@ -800,3 +800,8 @@ Expressive 2.0 will use zend-expressive-zendviewrenderer 1.3+ if that renderer
 is chosen. Starting with 1.3.0 of that renderer, you may now pass a boolean
 `false` value for the `layout` variable when calling either `addDefaultParam()`
 or `render()` on the renderer instance in order to disable the layout.
+
+## Twig renderer changes
+
+Expressive 2.0 will use zend-expressive-twigrenderer 1.3+ if that renderer
+is chosen. Starting with 1.3.0 of that renderer, Twig 2.1+ is now supported.