fix: import multiple segmentations from coco format json

[Ljung323]

Issue

https://github.com/HumanSignal/label-studio-sdk/issues/246

Changes

• d5287ad pass all segmentation items to create_segmentation function
• 3b76299 fix the content of help

Test

original image with segmentation

before change: only 1 section is imported

after change: all 5 sections are imported