[css-backgrounds] Properly define 'border image area'

[SebastianZ]

A definition of the border image area is available under the border-image-width property, though all references to it link to a sentence under the border-image-outset property.

It would be better if the border image area was described in its own section and only referenced the properties affecting it.

Sebastian

[fantasai]

I've fixed the cross-links, but I don't think this term needs its own section.