Add documentation surrounding [data-qa-selector]

What does this MR do?

Follow-up from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28906 and https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14623

Add documentation surrounding the data: { qa_selector: 'selector' } method of identification versus the old .qa-selector method of identification.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content