Improve contextType docs

See confusion in https://github.com/facebook/react/issues/13969.
Specific action items:

• Clarify .contextType is available only since 16.6 (and corresponding RN/Expo release)
• Clarify you may not be seeing it due to circular deps, and best practice is to declare it in a separate file
• People sometimes get confused thinking that createContext value is the one you need to "update", we need to explain it's driven by state
• Need more coherent doc structure

先閱讀現有的 contextType 文件，以及 issue 13969 中描述的困惑。釐清 React 16.6 的可用性、循環相依性的相關指引，以及由狀態驅動的 createContext 用法，然後將內容重新整理成更連貫的結構；文件應涵蓋列出的每一項困惑。