Update documentation for previous foreground user

The interactive headless system user counts as being a foreground user,
and now can be returned when asking for the previous foreground user.
This is a change from the previous year, and isn't obvious, so
documentation needs to be udpated.

Test: N/A just documentation
Bug: 412630247
Flag: EXEMPT comments
Change-Id: I876b69fb4a1c16c29d0373a01d3601783933188e