models description is non-deterministic

[baywet]

When a model has no description itself and is referred from a couple of path items, the description is non-deterministic (race condition) depending on which path item gets processed first.

This is because we default back to the path item description when a model has no description, which we shouldn't do when that model is a referenced schema.

This behaviour impacts sdk consumers in the sense that they get inaccurate descriptions, but more importantly it impacts us by generating larger diffs in weekly generations, making changes longer to process.