Update stock items article after PR review
Update stock items article after PR review

This clarifies some information about `Spree::StockItem`s after a
review by kennyadsl. Specifically, it confirms how many `StockItem`s
should exist if there are multiple `StockLocation`s, and it details how
backorderable stock items are handled.

pervino/solidus,pervino/solidus,pervino/solidus,pervino/solidus