libbinder: 'deprecate' note on BinderService

Asked recently about this, and I've wanted to delete it for a long time
(although there are quite a few users of it). Instead, documenting best
practices and also why this should be removed, for now.

Bug: N/A
Test: N/A
Change-Id: I9476f0bda28940be8c7855b6fcec299a9efd84b1