Merge "More keyguard docs" into sc-v2-dev am: 3dcea69858 am: 8cb32bb31d

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/16033060

Change-Id: I38f3e7e4f5c612b36f89c92250644f5dbd9412d9