(Obsoleted:) default.settings.php should include fast 404 enabled option

Problem/Motivation

In some cases it is possible that the fast_404 enabled status variable is either set to FALSE in the config or is left out entirely.

When this happens, it's not immediately obvious why fast_404 is not working, especially since these options don't appear to get configurable in the UI. You have to know to look in that particular configuration for the value.

One such case where I've noticed this happening is while using the config_split module and looking at an environment that does not explicitly define system.performance settings. So for example, suppose you have three environments: 1. Production 2. Staging 3. Development. But also assume you only have system.performance covered by two of the environments:
./config/sync/www-staging/system.performance.yml
./config/sync/www-production/system.performance.yml

If you navigate to admin/config/development/performance in your development environment and then click the "Save configuration" button, and then go to admin/config/development/configuration/single/export to export system.performance you get this:

cache:
  page:
    max_age: 0
css:
  preprocess: false
js:
  preprocess: false

In hindsight, this may be a slight oversight on my part not having a definition for system performance for my development environment, but this is just one example of a scenario where something like that could happen. That value could get set to FALSE by someone else and when people go to activate it, they struggle to identify why it's not working.

Proposed resolution

Add a commented out line to the default.settings.php showing how to explicitly enable fast 404, as this file already serves as a sort of documentation for the other configurable values for this functionality.

Remaining tasks

TBD

User interface changes

None

API changes

None

Data model changes

None

Release notes snippet

TBD