Bug 1384980 - Add a doc comment for apz.keyboard.enabled. r=botond

MozReview-Commit-ID: 5pQXqmUzxJQ

--HG--
extra : rebase_source : c14735e1fde801b8ee9eb388006e7012627ab33f

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -285,6 +285,10 @@ typedef GenericFlingAnimation FlingAnimation;
  * reflect the value of the async scroll offset and async zoom at the last time
  * SampleCompositedAsyncTransform() was called.
  *
+ * \li\b apz.keyboard.enabled
+ * Determines whether scrolling with the keyboard will be allowed to be handled
+ * by APZ.
+ *
  * \li\b apz.keyboard.passive-listeners
  * When enabled, APZ will interpret the passive event listener flag to mean
  * that the event listener won't change the focused element or selection of