Follow-up from "WIP: Documentation for sharing groups with groups"

The following discussion from !19962 (merged) should be addressed:

• @eread started a discussion: (+2 comments)

  > [Introduced](https://gitlab.com/gitlab-org/gitlab/issues/18328) in GitLab 12.5.
  
  Similarly to [sharing a project with a group](#sharing-a-project-with-a-group),
  you can share a group with another group to give direct group members (not
  inherited members) access to the shared group.
  
  ### Enabling this feature
  
  This feature comes with the `:share_group_with_group` feature flag disabled by default.
  To enable it, ask a GitLab administrator with Rails console access to run the following command:
  
  ```ruby
  Feature.enable(:share_group_with_group)
  ```

  This is more the style now. Can we add anything about why it's off by default? We provide some guidance on documenting feature flags here: https://docs.gitlab.com/ee/development/documentation/styleguide.html#feature-flags.