[java-firestore] Docs: Code snippet for City class does not match Android Java version

#### Environment details

Documentation on https://firebase.google.com/

#### Steps to reproduce

1. Open the [Add data to Cloud Firestore](https://firebase.google.com/docs/firestore/manage-data/add-data) page in the docs.
2. Compare the "Java" and "Java Android" code snippets under [Custom objects](https://firebase.google.com/docs/firestore/manage-data/add-data#custom_objects)
3. Notice that there a significant inconsistencies, and that both versions only show part of the class.

#### External references such as API reference guides

- https://firebase.google.com/docs/firestore/manage-data/add-data#custom_objects
- https://firebase.google.com/docs/firestore/query-data/get-data#custom_objects

#### Any additional information below

Related issue on Android snippet https://github.com/firebase/snippets-android/issues/322

See pull request https://github.com/googleapis/java-firestore/pull/877

检查 custom_objects 锚点处的“Add data to Cloud Firestore”页面，并比较其中的 Java 和 Java Android 代码片段，然后查看相关的 Android issue 和 java-firestore pull request 877。完成的标准是 City 类示例保持一致，并且每个代码片段都显示完整的类。

由索引模型根据 Issue 内容生成。