libbinder: Document future direction of getService.

I've had a CL a long time which did this here:
https://android-review.googlesource.com/c/platform/frameworks/native/+/1225405/

However, I am not sure how to get confidence in it, and it changes
threading structure, scheduling structure, etc... So I don't think it's
quite the right time to merge it yet.

Bug: n/a
Test: n/a
Change-Id: I83549b920a470387484245587a4714b69765f03f