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]

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Latest commit: 0fcf363

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