Describe the supported version of group-level variables API in document

What does this MR do?

This MR fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/36543#note_37876984

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

We should mention this earlier. This MR catches it up.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• Documentation created/updated
• [-] API support added
• [-] Tests added for this feature/bug
• Review
  • [-] Has been reviewed by UX
  • [-] Has been reviewed by Frontend
  • [-] Has been reviewed by Backend
  • [-] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/36543#note_37876984

Close https://gitlab.com/gitlab-org/gitlab-ce/issues/36543