isVisibleBackgroundUsersSupported is only for full

Clarifies that isVisibleBackgroundUsers* is specifically about FULL
users. It is totally unrelated to other background visible users.
In particular, its value has nothing to do with the existence of a
Communal Profile.

Bug: 323391957
Test: N/A. Just javadoc.
Change-Id: I1c970ecc36bfff2e298ee9eac15c343ba9e01afd