Document why usernames need to match for chatops

What does this MR do and why?

This MR adds some context about why Ops and GitLab.com usernames need to match when setting up chatops access. Because there are no technical requirements for these usernames to match there can be confusion about why this additional step is required.

Matching usernames are required to reduce the administrative effort of running multiple platforms and help with tasks including managing access requests and offboarding.

Screenshots or screen recordings

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.