Add comments stating the configs for visible background users are not

ready for production

Auto's concurrent multi user on multi display work based on the visible
background user model is in an incomplete state. Some system services
are not compatible with this user model yet which could lead to
potential security/privacy/safety issues.

Bug: 288205940
Test: m
Change-Id: I7f90599c985b9c29a6b8950914b10f31d27bc61b