Document dependencies between StringTemplateTrait and InstanceConfigT…

…rait.

Long term I'd like to remove this dependency, but today is not that day.

Refs #9827

src/View/StringTemplateTrait.php

@@ -17,6 +17,10 @@
 /**
  * Adds string template functionality to any class by providing methods to
  * load and parse string templates.
+ *
+ * This trait requires the implementing class to provide a `config()`
+ * method for reading/updating templates. An implementation of this method
+ * is provided by `Cake\Core\InstanceConfigTrait`
  */
 trait StringTemplateTrait
 {