docs: add information on when not to use tree-shakable providers

[sreevani-ship-it]

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
[x] Documentation content changes
[ ] angular.io application / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Does not have documentation for when not to use tree-shakable injector format.
Issue Number: 23621

What is the new behavior?

Added a note explaining when not to use tree-shakable provider format.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[mary-poppins]

You can preview cc522eb at https://pr23634-cc522eb.ngbuilds.io/.

[jenniferfell]

Looks okay to me.

@StephenFluin Please review. If okay, approve and mark for merge. Thanks!

[StephenFluin]

@chembu This text is not correct. Any provider can be overridden, but they must be overriden using the traditional providers array syntax.

[mary-poppins]

You can preview 33a00e2 at https://pr23634-33a00e2.ngbuilds.io/.

[mary-poppins]

You can preview e8b828d at https://pr23634-e8b828d.ngbuilds.io/.

[trotyl]

What's providers[] array? Should be providers array for property name or providers: [] array for usage or Provider[] for type annotation?

[sreevani-ship-it]

Providers: [] array syntax. Updated note to clarify it.

[mary-poppins]

You can preview ff07227 at https://pr23634-ff07227.ngbuilds.io/.

[mary-poppins]

You can preview a04b509 at https://pr23634-a04b509.ngbuilds.io/.

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