docs(guide/introduction): remove ambiguous "code-behind" jargon

This commit tries to remove the jargon and explain in plain English what
it means to add "code-behind" via a directive.

Closes angular#9684

Author: petebacondarwin

docs/content/guide/introduction.ngdoc

@@ -33,7 +33,7 @@ browser new syntax through a construct we call directives. Examples include:
   * Data binding, as in `{{}}`.
   * DOM control structures for repeating/hiding DOM fragments.
   * Support for forms and form validation.
-  * Attaching code-behind to DOM elements.
+  * Attaching new behavior to DOM elements, such as DOM event handling.
   * Grouping of HTML into reusable components.