Reword USER_TYPE_PACKAGE_WHITELIST_MODE_LOG

UserSystemPackageInstaller's log mode doesn't currently
log each non-whitelisted app's start (although it may
in the future), so the documentation should be updated.
It should be purposefully loose since the exact logging
is likely to change in the future.

Test: N/A. Just documentation.
Change-Id: Id7c46339d2b884d01e1e2a32d619a3a03a33958e