[Java][Docs] Compare/contrast the Netty and Unsafe memory backends

We should compare why you might want to use each.

Are there benchmarks in the Java benchmark suite that might also be useful?

**Reporter**: [David Li](https://issues.apache.org/jira/browse/ARROW-17300) / @lidavidm

_{**Note**: *This issue was originally created as [ARROW-17300](https://issues.apache.org/jira/browse/ARROW-17300). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

首先找到关于 Netty 和 Unsafe 内存后端的 Java 文档，然后检查 Java 基准测试套件，寻找相关比较。记录每个后端更适用的情况，并包含有用的基准测试参考；当文档清晰地比较两种选择及其权衡时，即表示完成。

由索引模型根据 Issue 内容生成。