Update privilege escalation wording

What does this MR do?

This MR applies the suggestion from @rcoleman-gitlab in !215843 (merged) to clarify the privilege escalation wording.

Change: Updates the description to clarify that composite identity prevents privilege escalation by either the human user or the service account, not just the human user.

From:

preventing privilege escalation for the human user.

To:

preventing privilege escalation by either the human user or the service account.

Related issues

Follow-up from !215843 (merged) (comment !215843 (comment 2951350140))

Author's checklist

• Follow the Documentation guidelines.
• Follow the Style Guide.