Briefly explain how to split element definition

Author: dandv

PRIMER.md

@@ -467,7 +467,8 @@ Example:
 We say that an element definition has an imperative and declarative portion. The imperative
 portion is the call to `Polymer({...})`, and the declarative portion is the `<dom-module>`
 element. The imperative and declarative portions of an element's definition may be placed
-in the same html file or in separate files.
+in the same HTML file or in separate files. They are matched mased on the `id` and `is`
+properties having the same value.
 
 **NOTE:** Defining an element in the main html document is not currently supported.