Even if the user (usually accidentally) sets a thread count of zero or
negative. The reporter in the bug report did
QThread::idealThreadCount() - 1 on a 1 CPU system...
Drive-by add to the documentation and the missing #include.
Pick-to: 6.1 6.0
Fixes: QTBUG-93007
Change-Id: I6cdea00671e8479b9c50fffd167807d14e030154
Reviewed-by: Samuel Gaist <samuel.gaist@idiap.ch>
Reviewed-by: Sona Kurazyan <sona.kurazyan@qt.io>