[PR] Docs: Add note about Maybe-typed object properties
[PR] Docs: Add note about Maybe-typed object properties

Summary:
Fixes #6768.
Pull Request resolved: https://github.com/facebook/flow/pull/6771

Reviewed By: fishythefish

Differential Revision: D9584977

Pulled By: mrkev

fbshipit-source-id: 1cdeccef7d4c70695581b5a861ad12ed8d596cd2

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