Add application note for explicit presence tracking. (#7390)
Add application note for explicit presence tracking. (#7390)

This includes instructions for enabling explicit presence tracking in proto3, and also explains implicit presence. The language examples were written by inspection, and not individually tested.
google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf