Merge "Update gainmap documentation." into udc-dev am: 19573fe4

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/21701716



Change-Id: I56365c0469bb4e547ac21814a2a8f5148595189e
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>