Merge branch 'fix-api-doc-grouplevel_variables' into 'master'

doc: add api group level variables 255 chars limit

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/163892



Merged-by: Lysanne Pinto <lpinto@gitlab.com>
Approved-by: Lysanne Pinto <lpinto@gitlab.com>
Co-authored-by: Marcus Jung <mfuegy@gmx.de>