Document Backorder support in MSI.

Supported currently:
Backorders - merchants can enable backorders for inventory. Document the process, best practices, caveats.

Doc pages:

• for some setting info - https://github.com/magento-engcom/msi/wiki/MSI-Global-Settings
• will create a new page

NOTES:

• If backorders enabled, salable qty could potentially be infinite!
• If backorders enabled with Allow Qty Below 0, the threshold amount is ignored.
• Looking at roadmap, SSA doesn't look to work on backorders (possible Milestone 3)?

If all sources are in backorder, does it offer the lowest negative quantity?