Bug 1353798 - [devtools] Add documentation for service worker timings r=devtools-reviewers,ochameau

Differential Revision: https://phabricator.services.mozilla.com/D196389

Author: bomsy

devtools/docs/user/network_monitor/request_details/index.rst

@@ -457,6 +457,31 @@ New in Firefox 71, the *Server Timing* section lists any information provided in
 
 The header takes a series of descriptions and durations, which can be anything you like. In the above screenshot for example, the highlighted request's ``Server-Timing`` header contains 4 items — *data*, *markup*, *total*, and *miss*.
 
+Service Worker Timing
+~~~~~~~~~~~~~~~~~~~~~
+
+The *Service Worker Timing* section lists the information relating to the specific service worker request. The metrics include Startup, Dispatch fetch and Handle fetch.
+
+.. image:: network-service-worker-timings.png
+  :class: border
+
+.. list-table::
+  :widths: 20 80
+  :header-rows: 0
+
+
+  * - Name
+    - Description
+
+  * - Startup
+    - Time taken to launch the service worker, this is only indicated if the launch starts after the fetch event has already been dispatched.
+
+  * - Dispatch fetch
+    - Time taken from when a fetch event is triggered to just before it starts getting handled by the service worker.
+
+  * - Handle fetch
+    - Time taken to by the service worker to handle the fetch event.
+
 
 .. _network-monitor-request-details-security-tab: