Popover > Document close button usage

Description

As per gitlab-org/gitlab-ui!2630 (merged), we are introducing a close option to our popover component. We need to document its usage.

Checklist

Make sure the following are completed before closing the issue:

1. Assign the correct component label to this issue.
2. Create an MR with the additions or updates needed.
3. When applicable, create a new Pajamas issue using the "Figma update" issue template to update the component in Figma. Bring the issue to your team planning session for prioritization and scheduling. Mark the issue as related to this one.
4. When applicable, create an MR in GitLab UI to update the component. If you do not have capacity or are unable to update the component directly, create a new GitLab UI issue using the "Component" issue template. Bring the issue to your team planning session for prioritization and scheduling. Mark the issue as related to this one.
5. When introducing a major or breaking change, communicate the changes within the Engineering Week in Review and UX Weekly meeting.
6. 🎉 Congrats, you made it! You can now close this issue.