Rosik/access control

Summary

• access_control.md: improve db_user
• access_control.md: make db_admin a role
• access_control.md: revise sql_examples
• access_control.md: add missing quotes
• access_control.md: shorten db_admin

---

• Close #...
• Cherry-pick to: all 24.2, 24.3, 24.4
• Staging: https://docs.binary.picodata.io/picodata/rosik_access_control/