Repository: opencv
Issue: Doxygen comment about calcOpticalFlowFarneback() is incorrect
Labels: category: video,category: documentation,affected: 3.4

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##### System information (version)
<!-- Example
- OpenCV => 4.2
- Operating System / Platform => Windows 64 Bit
- Compiler => Visual Studio 2017
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- OpenCV => 4.5.2
- Operating System / Platform => Windows 64 Bit
- Compiler => Visual Studio 2017

##### Detailed description

Here is Doxygen comment about void calcOpticalFlowFarneback()

```.cpp
/** @brief Computes a dense optical flow using the Gunnar Farneback's algorithm.

@param prev first 8-bit single-channel input image.
@param next second input image of the same size and the same type as prev.
@param flow computed flow image that has the same size as prev and type CV_32FC2.
@param pyr_scale parameter, specifying the image scale (\<1) to build pyramids for each image;
pyr_scale=0.5 means a classical pyramid, where each next layer is twice smaller than the previous
one.
@param levels number of pyramid layers including the initial image; levels=1 means that no extra
layers are created and only the original images are used.
```

This says "@param levels number of pyramid layers including the initial image; levels=1 means that no extra layers are created and only the original images are used.", but levels works "number of pyramid layers **ex**cluding the initial image; levels=**0** means that no extra layers are created and only the original images are used."

Here is working log with levels=4, there are pyramid layers k=0,1,2,3 and 4.
```
calc:1384 levels:4/4
    (*: [levels with (min(height, width) / power(2,k)) < 32] are ignored, but doxygen comment doesn't says about it)
calc:1419 k:4 scale:0.0625 39x32
calc:1419 k:3 scale:0.125 78x64
calc:1419 k:2 scale:0.25 156x128
calc:1419 k:1 scale:0.5 313x256
calc:1419 k:0 scale:1 626x512
```

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##### Steps to reproduce

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