Rewriting abstract (w3c#1494)

* Rewriting abstract

* Rolling the 'extensible' bit into a previous line

* Updating abstract

docs/index.bs

@@ -13,11 +13,11 @@ Repository: w3c/ServiceWorker
 Group: serviceworkers
 !Tests: <a href=https://github.com/web-platform-tests/wpt/tree/master/service-workers>web-platform-tests service-workers/</a> (<a href=https://github.com/web-platform-tests/wpt/labels/service-workers>ongoing work</a>)
 Status Text: This is a living document. Readers need to be aware that this specification may include unimplemented features, and details that may change. <a href="https://w3c.github.io/ServiceWorker/v1/">Service Workers 1</a> is a version that is advancing toward a W3C Recommendation.
-Abstract: This specification describes a method that enables applications to take advantage of persistent background processing, including hooks to enable bootstrapping of web applications while offline.
+Abstract: The core of this specification is a worker that wakes to receive events. This provides an event destination that can be used when other destinations would be inappropriate, or no other destination exists.
 Abstract:
-Abstract: The core of this system is an event-driven <a>Web Worker</a>, which responds to events dispatched from documents and other sources. A system for managing installation, versions, and upgrades is provided.
+Abstract: For example, to allow the developer to decide how a page should be fetched, an event needs to dispatch potentially before any other execution contexts exist for that origin. To react to a push message, or completion of a persistent download, the context that originally registered interest may no longer exist. In these cases, the service worker is the ideal event destination.
 Abstract:
-Abstract: The service worker is a generic entry point for event-driven background processing in the Web Platform that is <a href="#extensibility">extensible by other specifications</a>.
+Abstract: This specification also provides a [=handle fetch|fetch event=], and a [[#cache-objects|request and response store]] similar in design to the HTTP cache, which makes it easier to build offline-enabled web applications.
 Markup Shorthands: css no, markdown yes
 </pre>