docs: clarify a possible error case

[ynden]

If we use a Typescript version >= 2.7, we get an error:

Property 'hero' has no initializer and is not definitely assigned in the constructor.ts(2564)

We need to inform the user of a possible solution. This is what I described.

Another solution would be to set "strictPropertyInitialization": false in tsconfig.json.

PR Checklist

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• 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
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What is the new behavior?

Does this PR introduce a breaking change?

• Yes
• No

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[ynden]

@googlebot I signed it!

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