Move Server Timing info to Configuration guide [ci-skip]

Follow-up to rails#48221.

The Active Support Instrumentation guide is about the
`ActiveSupport::Notifications` API.  Therefore, move the information
about `config.server_timing` and the `ActionDispatch::ServerTiming`
middleware to the existing sections in the Configuration guide.

Author: jonathanhefner

guides/source/active_support_instrumentation.md

@@ -11,7 +11,6 @@ After reading this guide, you will know:
 
 * What instrumentation can provide.
 * How to add a subscriber to a hook.
-* How to view timings from instrumentation in your browser.
 * The hooks inside the Rails framework for instrumentation.
 * How to build a custom instrumentation implementation.
 
@@ -85,17 +84,6 @@ end
 [`ActiveSupport::Notifications.monotonic_subscribe`]: https://api.rubyonrails.org/classes/ActiveSupport/Notifications.html#method-c-monotonic_subscribe
 [`ActiveSupport::Notifications.subscribe`]: https://api.rubyonrails.org/classes/ActiveSupport/Notifications.html#method-c-subscribe
 
-View Timings from Instrumentation in Your Browser
--------------------------------------------------
-
-Rails implements the [Server Timing](https://www.w3.org/TR/server-timing/) standard to make timing information available in the web browser. To enable, edit your environment configuration (usually `development.rb` as this is most-used in development) to include the following:
-
-```ruby
-config.server_timing = true
-```
-
-Once configured (including restarting your server), you can go to the Developer Tools pane of your browser, then select Network and reload your page. You can then select any request to your Rails server, and will see server timings in the timings tab. For an example of doing this, see the [Firefox Documentation](https://firefox-source-docs.mozilla.org/devtools-user/network_monitor/request_details/index.html#server-timing).
-
 Rails Framework Hooks
 ---------------------
 

guides/source/configuring.md

@@ -547,8 +547,9 @@ for more information and alternative configuration methods.
 
 #### `config.server_timing`
 
-When `true`, adds the [ServerTiming middleware](#actiondispatch-servertiming)
-to the middleware stack
+When `true`, adds the [`ServerTiming` middleware](#actiondispatch-servertiming)
+to the middleware stack. Defaults to `false`, but is set to `true` in the
+default generated `config/environments/development.rb` file.
 
 #### `config.session_options`
 
@@ -768,8 +769,12 @@ Rails.application.config.host_authorization = {
 
 #### `ActionDispatch::ServerTiming`
 
-Adds metrics to the `Server-Timing` header to be viewed in the dev tools of a
-browser.
+Adds the [`Server-Timing`][] header to the response, which includes performance
+metrics from the server. This data can be viewed by inspecting the response in
+the Network panel of the browser's Developer Tools. Most browsers provide a
+Timing tab that visualizes the data.
+
+[`Server-Timing`]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Server-Timing
 
 #### `ActionDispatch::SSL`