Expand doc for APP_INITIALIZER token

[jbogarthyde]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• angular.io application / infrastructure changes
• Other... Please describe:

What is the current behavior?

The description of https://angular.io/api/core/APP_INITIALIZER is brief and uninformative.

Issue Number: #34703

What is the new behavior?

Expands the API doc description.
Needs an example or pointer to additional doc to be really useful.
The subject does not appear to be covered anywhere in the current doc, but lots of external discussion.

Does this PR introduce a breaking change?

• Yes
• No

[AndrewKushnir]

👍 LGTM, thanks for the update.

I think it'd be great to add an example in the followup PR on how this token can be used.

[mary-poppins]

You can preview e738e32 at https://pr34760-e738e32.ngbuilds.io/.

[CaerusKaru]

I think we should say provide one or more functions to be executed. Those functions then get injected at application startup and run. This verbiage is similar to how the value is actually used in code:

{
   provide: APP_INITIALIZER,
   useValue: () => {}
}

[CaerusKaru]

This whole example has me a little confused, mostly I think just by the wording. If I get your intention correctly, maybe this would be more direct?

You can, for example, create a factory function that loads language data or an external configuration, and provide that function to the APP_INITIALIZER token. 
That way, the function is executed during the application bootstrap process, and the needed data is available on startup.

[jbogarthyde]

Thank you!

[mary-poppins]

You can preview 75831e2 at https://pr34760-75831e2.ngbuilds.io/.

[mary-poppins]

You can preview 5a7de1a at https://pr34760-5a7de1a.ngbuilds.io/.

[angular-automatic-lock-bot]

This issue has been automatically locked due to inactivity.
Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}