[Docs] Add FAQ about COCO AP or AR =-1  (#5313)
[Docs] Add FAQ about COCO AP or AR =-1  (#5313)

* [Docs] add faq about coco evaluation

* [Docs] add faq about coco evaluation

* [Docs] add FAQ about coco result=-1
open-mmlab/mmdetection,open-mmlab/mmdetection