Add warning for public projects shared with "Internal" group

Context

Follow-up to !91909 (comment 1021063302)

In !91909 (merged) we added a warning message for public projects shared with private groups. See https://docs.gitlab.com/ee/user/project/members/share_project_with_groups.html#share-a-public-project-with-private-group for more context.

We should also add this warning for public projects shared with internal groups and update the docs to reflect this.