docs(vpc): update VPC documentation in README and API

[rix0rrr]

Try to address common gotchas with the VPC component in the
package documentation.

Addresses part of #3704.

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[nija-at]

1. Minor rephrase on 1st line: "The number of availability zones that are available depend on ..."

2. I was aware that AZs are dependent on the region but didn't know they were dependent on the account. Are they?

3. Link to this page would be useful - https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-regions-availability-zones.html#using-regions-availability-zones-describe

[rix0rrr]

2. Yes, although this mostly only happens when new AZs are introduced and there is a whitelisting period.

[nija-at]

What happens if this assumption is not true? Specify that.

[rix0rrr]

Fixes #3810.

[mergify]

Pull Request Checklist

• Testing

• Unit test added (prefer to add a new test rather than modify existing tests)
• CLI change? Re-run/add CLI integration tests

• Documentation

• Inline docs: make sure all public APIs are documented (copy & paste from official AWS docs)
• README: update module README
• Design: for significant features, follow the design process

• Title uses the format type(scope): text

• Type: fix, feat, refactor go into CHANGELOG, chore is hidden
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• Style: use all lower-case, do not end with a period

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• Breaking?: last paragraph: BREAKING CHANGE: <describe what changed + link for details>

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• PR title type(scope): text
  • type: fix, feat, refactor go into CHANGELOG, chore is hidden
  • scope: name of module without aws- or cdk- prefix or postfix (e.g. s3 instead of aws-s3-deployment)
  • text: use all lower-case, do not end with a period, do not include issue refs
• PR Description
  • Rationale: describe rationale of change and approach taken
  • Issues: indicate issues fixed via: fixes #xxx or closes #xxx
  • Breaking?: last paragraph: BREAKING CHANGE: <describe what changed + link for details>
• Testing
  • Unit test added. Prefer to add a new test rather than modify existing tests
  • CLI or init templates change? Re-run/add CLI integration tests
• Documentation
  • README: update module README to describe new features
  • API docs: public APIs must be documented. Copy from official AWS docs when possible
  • Design: for significant features, follow design process