Update Asset Pipeline Guide to describe how Sprockets 4 works

Fixes https://github.com/rails/rails/issues/43203

To test this, I
- Uninstalled all Rails gems
- `gem install rails --pre`
- `rails new asset_test`
- Made these changes, to use Sprockets for JS instead of importmaps: 7d961765c0
- Confirmed that no source map is rendered by default
- Set `config.assets.debug = true`, and confirmed that a source map is now rendered

guides/source/asset_pipeline.md

@@ -508,8 +508,7 @@ wouldn't understand ERB and therefore you would run into problems.
 In Development
 --------------
 
-In development mode, assets are served as separate files in the order they are
-specified in the manifest file.
+In development mode, assets are served as a concatenated file.
 
 This manifest `app/assets/javascripts/application.js`:
 
@@ -522,13 +521,9 @@ This manifest `app/assets/javascripts/application.js`:
 would generate this HTML:
 
 ```html
-<script src="/assets/core.js?body=1"></script>
-<script src="/assets/projects.js?body=1"></script>
-<script src="/assets/tickets.js?body=1"></script>
+<script src="/assets/application-728742f3b9daa182fe7c831f6a3b8fa87609b4007fdc2f87c134a07b19ad93fb.js"></script>
 ```
 
-The `body` param is required by Sprockets.
-
 ### Raise an Error When an Asset is Not Found
 
 If you are using sprockets-rails >= 3.2.0 you can configure what happens
@@ -553,43 +548,26 @@ config.assets.digest = false
 
 When this option is true, digests will be generated for asset URLs.
 
-### Turning Debugging Off
+### Turning Source Maps On
 
-You can turn off debug mode by updating `config/environments/development.rb` to
+You can turn on source maps by updating `config/environments/development.rb` to
 include:
 
 ```ruby
-config.assets.debug = false
+config.assets.debug = true
 ```
 
-When debug mode is off, Sprockets concatenates and runs the necessary
-preprocessors on all files. With debug mode turned off the manifest above would
-generate instead:
-
-```html
-<script src="/assets/application.js"></script>
-```
+When debug mode is on, Sprockets will generate a Source Map for each asset. This
+allows you to debug each file individually in your browser's developer tools.
 
 Assets are compiled and cached on the first request after the server is started.
 Sprockets sets a `must-revalidate` Cache-Control HTTP header to reduce request
 overhead on subsequent requests - on these the browser gets a 304 (Not Modified)
 response.
 
-If any of the files in the manifest have changed between requests, the server
+If any of the files in the manifest change between requests, the server
 responds with a new compiled file.
 
-Debug mode can also be enabled in Rails helper methods:
-
-```erb
-<%= stylesheet_link_tag "application", debug: true %>
-<%= javascript_include_tag "application", debug: true %>
-```
-
-The `:debug` option is redundant if debug mode is already on.
-
-You can also enable compression in development mode, and
-disable it on-demand as required for debugging.
-
 In Production
 -------------