docs: Add FFI docs to contributor guide

[andygrove]

Which issue does this PR close?

N/A

Rationale for this change

FFI is complex. I'd like it documented so that I do not need to keep re-learning how this works. I hope this will be useful for other contributors as well.

What changes are included in this PR?

New docs.

How are these changes tested?

[wForget]

Good job, thanks @andygrove

[mbutrovich]

Just to clarify: does "ownership transfer" mean "the code responsible for cleaning up the originally allocated data" i.e., the data still resides possibly on the other side of a JNI boundary, or does it mean that "data was deep copied across the JNI boundary so the original copy is not necessary?"

[andygrove]

Just to clarify: does "ownership transfer" mean "the code responsible for cleaning up the originally allocated data" i.e., the data still resides possibly on the other side of a JNI boundary

Yes. The Arrow C Data Interface is designed to transfer ownership of data across a JNI boundary by passing a pointer to a callback to delete the data. For example, the ArrowArray struct has:

  // Release callback
  void (*release)(struct ArrowArray*);

[andygrove]

Just to clarify: does "ownership transfer" mean "the code responsible for cleaning up the originally allocated data" i.e., the data still resides possibly on the other side of a JNI boundary

Yes. The Arrow C Data Interface is designed to transfer ownership of data across a JNI boundary by passing a pointer to a callback to delete the data. For example, the ArrowArray struct has:

  // Release callback
  void (*release)(struct ArrowArray*);

I pushed a commit to clarify this in the docs.