Merge "RefBase: document 0 strong count, wp<>::promote" into main am: d217d8dc16

Original change: https://android-review.googlesource.com/c/platform/system/core/+/3497753

Change-Id: I526aa8b5f441586a4c3c55b736458892769b7708
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>