RBAC Docs Updates

[Amruta-Ranade]

Amruta Ranade commented:

Based on the following docs/issues, update RBAC docs across the docset:

• https://docs.google.com/document/d/17eY5UJnExEQ-uPnA5EyGH14i3h3szzHIzxbOfdvmRCc/edit#heading=h.gs4zsc68k3h
• sql: Add ownership concept for objects cockroach#51856
• Establish equivalency between the keywords ROLE and USER #7063
• sql: introduce the new role option CREATELOGIN cockroach#50601
• https://docs.google.com/document/d/1WVgulIQX_Uf3SybHAZI8O-HQSfLOQq4G7Gm0Hw9NIt0

Jira Issue: DOC-629