add docs regarding observers and computed properties (ref emberjs#774)

packages/ember-runtime/lib/ext/function.js

@@ -51,7 +51,10 @@ if (Ember.EXTEND_PROTOTYPES) {
         });
 
     Make sure you list these dependencies so Ember.js knows when to update
-    bindings that connect to a computed property.
+    bindings that connect to a computed property. Changing a dependency
+    will not immediately trigger an update of the computed property, but
+    will instead clear the cache so that it is updated when the next `get`
+    is called on the property.
 
     Note: you will usually want to use `property(...)` with `cacheable()`.
 

packages/ember-runtime/lib/mixins/observable.js

@@ -61,6 +61,11 @@ var get = Ember.get, set = Ember.set;
   This will call the `targetAction` method on the `targetObject` to be called
   whenever the value of the `propertyKey` changes.
   
+  Note that if `propertyKey` is a computed property, the observer will be 
+  called when any of the property dependencies are changed, even if the 
+  resulting value of the computed property is unchanged. This is necessary
+  because computed properties are not computed until `get` is called.
+  
   @extends Ember.Mixin
 */
 Ember.Observable = Ember.Mixin.create(/** @scope Ember.Observable.prototype */ {