Merge branch 'document_user_identities_field' into 'master'

Document pass_user_identities_to_ci_jwt usage

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/118231



Merged-by: Furkan Ayhan <furkanayhn@gmail.com>
Approved-by: Jon Glassman <jglassman@gitlab.com>
Approved-by: Furkan Ayhan <furkanayhn@gmail.com>
Reviewed-by: Joe Snyder <joe.snyder@kitware.com>
Reviewed-by: Avielle Wolfe <awolfe@gitlab.com>
Co-authored-by: Joseph Snyder <joe.snyder@kitware.com>