[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.*}

Start by locating the Java documentation for the Netty and Unsafe memory backends, then inspect the Java benchmark suite for relevant comparisons. Document when each backend is preferable and include useful benchmark references; done means the documentation clearly compares both choices and their trade-offs.

Written by the indexing model from the issue text.