Updating documentation: cookie used for sign-in

What does this MR do and why?

Describe in detail what your merge request does and why.

Updating documentation for cookies using during sign-in by adding info about the about_gitlab_active_user

related to: gitlab-com/marketing/digital-experience/buyer-experience#2300 (closed)

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.