[Java] MapWithOrdinal javadoc doesn't describe actual behaviour

MapWithOrdinal states that ordinals are recycled when keys are removed, it does not currently do this and grows unbounded

**Reporter**: [Ryan Murray](https://issues.apache.org/jira/browse/ARROW-8947) / @rymurr
#### PRs and other links:
- [GitHub Pull Request apache/arrow#7276](https://github.com/apache/arrow/pull/7276)

_{**Note**: *This issue was originally created as [ARROW-8947](https://issues.apache.org/jira/browse/ARROW-8947). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

從 MapWithOrdinal 的 Javadoc 開始，並檢視連結的 pull request apache/arrow#7276，以了解預期的文件變更。當 Javadoc 準確描述移除鍵時是否會重複使用序號，且不再與實作矛盾時，即表示完成。

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