Add documentation for bot comment for security approvals
What does this MR do and why?
This MR updates the documentation and adds a mention of bot comment under security approvals.
MR acceptance checklist
This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
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I have evaluated the MR acceptance checklist for this MR.
Related to #411656 (closed)