Add some more public doc for MacAddress

Bug: 140807677
Test: doc-only change
Original-Change: https://android-review.googlesource.com/1354447
Merged-In: I0f6e59eda42fd92ec34db0e9bc2d26d2e83d41d0
Change-Id: I0f6e59eda42fd92ec34db0e9bc2d26d2e83d41d0
1 file changed