docs: clarify optional input blocks

[deepunyk]

Summary

Fixes #2044 by documenting that optional: true belongs on the enclosing input block and applies to its child element, including multi_external_select.

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

Checks

• npm run build
• npm run lint
• npm run test:types
• git diff --check
• The full test suite reached 476 passing tests and one unrelated timeout in conversationContext; the isolated test passed on rerun.

[changeset-bot]

🦋 Changeset detected

Latest commit: 05d1a0b

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@slack/bolt	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR