[docs] CocoDetection, VOCDetection should specify format of targets

[vadimkantorov]

🐛 Describe the bug

And be very clear on the format of bounding boxes: xywh, are x/y zero-based or not and if width do/not include the final pixel

Complications about this are described in:

• https://ppwwyyxx.com/blog/2021/Where-are-Pixels/#Choices-of-Sampling-Grid
• https://bartwronski.com/2021/02/15/bilinear-down-upsampling-pixel-grids-and-that-half-pixel-offset/

COCO seems to have floating-point annotations, VOC seems to have integer-based annotations (0/1-based? inclusive/exclusive for final point?)

[datumbox]

Related discussion at #4745