Current implementation doesn't corresponding to this documentation.
The exact meaning of the return value depends on the threading framework used by OpenCV library:
TBB - Unsupported with current 4.1 TBB release. Maybe will be supported in future.
OpenMP - The thread number, within the current team, of the calling thread.
Concurrency - An ID for the virtual processor that the current context is executing on (0
for master thread and unique number for others, but not necessary 1,2,3,...).
GCD - System calling thread's ID. Never returns 0 inside parallel region.
C= - The index of the current parallel task.
SEE: setNumThreads, getNumThreads