Update behaviors order. Fixes #2144.

PRIMER.md

@@ -386,7 +386,7 @@ Polymer supports extending custom element prototypes with shared code modules ca
 
 A behavior is simply an object that looks very similar to a typical Polymer prototype.  It may define lifecycle callbacks, `properties`, `hostAttributes`, or other features described later in this document like `observers` and `listeners`.  To add a behavior to a Polymer element definition, include it in a `behaviors` array on the prototype.
 
-Lifecycle callbacks will be called on the base prototype first, then for each behavior in the order given in the `behaviors` array.   Additionally, any non-lifecycle functions on the behavior object are mixed into the base prototype (and will overwrite the function on the prototype, if they exist); these may be useful for adding API or implementing observer or event listener callbacks defined by the behavior, for example.
+Lifecycle callbacks will be called for each behavior in the order given in the `behaviors` array first, followed by the callback on the base prototype.   Additionally, any non-lifecycle functions on the behavior object are mixed into the base prototype unless a function of the same name already exists; these may be useful for adding API or default observer or event listener callbacks defined by the behavior, for example, while allowing the base prototype to provide a more specific implementation of the behavior API when necessary.
 
 Example: `highlight-behavior.html`