Document more errno(s) on ADnsHelper_ API am: 96aa4824b9

Original change: https://android-review.googlesource.com/c/platform/packages/modules/Connectivity/+/2899547

Change-Id: I5b8829b86432e8803fb57f8cf658afbef6d6e0a0
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>