Document overselling prevention setting

[kilbot]

Part of wcpos/roadmap#1

Document the new "Prevent overselling" feature:

• Where to find the setting: WP Admin > POS > Settings > Checkout
• What it does when enabled: blocks adding out-of-stock items to cart and rejects orders server-side if stock is insufficient
• Default behavior: disabled (existing behavior unchanged)
• How it interacts with WooCommerce's backorder settings
• Any error messages the cashier might see and what they mean

[kilbot]

This feature hasn't been implemented yet — closing PR #142. Reopening this issue to track for when the feature ships.