Also move some docs from asBackendZone() to systemTimeZone(), making
clear that the system zone object is current at the time of creation
and won't be updated if the system is reconfigured. Adapt some tests
to fail and make clear that the system is misconfigured if no valid
system zone is found.
[ChangeLog][QtCore][QTimeZone] If systemTimeZone() is unable to
identify a valid system time zone, it now produces a warning the first
time it encounters the problem.
Task-number: QTBUG-116017
Change-Id: Ia437d8a03ff3cbf2b2cd98e8a8c3aebe50c1ee32
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
9c0def81e4