Add information in regards to invalid approvals in the docs

What does this MR do and why?

Add information in regards to invalid approvals in the docs.

Invalid will be applied to MRs:

• when the author is the only eligible approver.
• when there is no eligible approver assigned to the rule.

Related issue: #334698 (closed)

Backend related MR: !87596 (merged)

Frontend related MR: !88941 (merged)

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