docs: clarify hierarchical injectors

[kapunahelewong]

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?

Issue Number: Closes #28508

What is the new behavior?

• Corrects @NgModule.providedIn to @NgModule providers array.
• Clarifies limiting where a service is available
• Specifies terms ModuleInjector and ElementInjector alongside current description.
• Improves some grammar by replacing passive voice with active.
• Adds new sections for @Optional(), @Self(), @SkipSelf(), and@Host()
• Adds new diagrams
• Adds new example for resolution modifiers

Does this PR introduce a breaking change?

• Yes
• No

Other information

7/31 To do: write e2e tests and check for flow

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[kapunahelewong]

Caretaker, this one's big (though is docs only) so if you need me, please let me know. Thank you!!!

[AndrewKushnir]

@kapunahelewong thanks for the update. Could you please rebase this PR on top of the latest master (since last rebase was 9 days agi), so we can run CI again to make sure it'd green? Thank you.

[mary-poppins]

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