Binder death recipients: clarify comments

These say "this should not happen" like it is memory
corruption or bad behavior, but code has been relying
on this a long time. Clarify language as it has gotten
out of date over time, and it was confusing me when
I was reading this code.

Bug: 405737267
Test: N/A
Change-Id: Iaa8367ed0313c5fb9410a5f76644d448e9db120a