[Java] Document how to set netty/unsafe allocators

[asfimport]

There are now 2 allocators available, one based on netty and one using unsafe apis. We should provide end-user documentation on which one is default and how to set and use each one.

Reporter: Bryan Cutler / @BryanCutler

Related issues:

• [Java] Separate Netty Memory to its own module (is related to)

_{Note: This issue was originally created as ARROW-9357. Please see the migration documentation for further details.}

[thisisnic]

This issue hasn't had activity in a long time. If it's still being worked on, please leave a comment. Otherwise, it will be closed on 23rd June.

Labelled Status: Stale-Warning for tracking.