diff --git a/src/config.coffee b/src/config.coffee
index 9061d9771..02b1ea1eb 100644
--- a/src/config.coffee
+++ b/src/config.coffee
@@ -222,9 +222,10 @@ class Config
   #
   # keyPath - The {String} name of the key to observe
   # options - An optional {Object} containing the `callNow` key.
-  # callback - The {Function} that fires when the value of the key is
-  #            changed. It is given a single argument, `value`, which is
-  #            the new value of `keyPath`.
+  # callback - The {Function} to call when the value of the key changes.
+  #            The first argument will be the new value of the key and the
+  #            second argument will be an {Object} with a `previous` property
+  #            that is the prior value of the key.
   #
   # Returns an {Object} with the following keys:
   #  :off - A {Function} that unobserves the `keyPath` when called.