Fix and complete decorator API doc

[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?

Fix existing doc to be in correct places to be rendered
Add top-level descriptions and property descriptions where missing.
Edit and correct descriptions.

• 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: #28710

What is the new behavior?

Does this PR introduce a breaking change?

• Yes
• No

Other information

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[brandonroberts]

Caretaker note: This is a docs only change and should not impact g3. Please run presubmit.

[AndrewKushnir]

Presubmit

[AndrewKushnir]

@jbogarthyde, @brandonroberts there was a conflict while merging this PR into patch branch, so I've merged it to master only. Could you please create a version of this PR against patch (7.2.x) branch? Thank you.

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