DAOS-XXX docs: Document BIO threading model

[tanabarr]

Doc-only: true

Steps for the author:

• Commit message follows the guidelines.
• Appropriate Features or Test-tag pragmas were used.
• Appropriate Functional Test Stages were run.
• At least two positive code reviews including at least one code owner from each category referenced in the PR.
• Testing is complete. If necessary, forced-landing label added and a reason added in a comment.

After all prior steps are complete:

• Gatekeeper requested (daos-gatekeeper added as a reviewer).

[github-actions]

Errors are Unknown component,Ticket number suffix is not a number. See https://daosio.atlassian.net/wiki/spaces/DC/pages/11133911069/Commit+Comments,Unable to load ticket data
https://daosio.atlassian.net/browse/DAOS-XXX

[tanabarr]

@NiuYawei @Michael-Hennecke would this be useful? I've copied it into two places as I'm not sure which is best, LMK what you think? it was primarily for my own understanding but I thought it might be useful as developer documentation