Merge branch 'doc-issue-export-field-closed' into 'master'

Document "Closed At" field in issue CSV export

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/156385



Merged-by: Diana Logan <dlogan@gitlab.com>
Approved-by: Diana Logan <dlogan@gitlab.com>
Co-authored-by: Katrin Leinweber <kleinweber@gitlab.com>