Fix approval segregation of duties documentation

What does this MR do?

This came up from the discussion on #287795 (comment 618934180)

Currently, these docs doesn't mention that there should be a protected branch the rule should target it.

All of this is very complex an we hope to fix it by #332830 (comment 594884698), but for now just fixing the docs.

Screenshots or Screencasts (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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Security

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• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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