docs(forms): Add JSDoc content for typed forms.

[dylhunn]

Update the JSDoc on forms model classes with more specific information about the new types.

(public-api review is spurious; no actual API changes were made.)

Issue: #13721

[atscott]

reviewed-for: public-api

[dylhunn]

merge-assistance: this is not a real public-api change; it's just docs that happen to trigger a minor update to one of the golden files.

[ngbot]

I see that you just added the action: merge label, but the following checks are still failing:
     status "ci/circleci: test_win" is failing
     status "google3" is pending
     status "pullapprove" is pending
     3 pending code reviews

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If you can't get the PR to a green state due to flakes or broken master, please try rebasing to master and/or restarting the CI job. If that fails and you believe that the issue is not due to your change, please contact the caretaker and ask for help.

[dylhunn]

This PR was merged into the repository by commit fce5063.

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