Github Import: inconsistent import of pull request assignees/reviewers

What does this MR do and why?

Updates documentation for Importer:GitHub

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #386714 (closed)