Improve UX guide on tooltips

What does this MR do?

Improves UX guide on tooltips per https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10182#note_26075373

Why was this MR needed?

There is a lack of guidance on when tooltips should be used. Specifically, there was no documentation saying that tooltips should always be used for “lonely” icons.

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Tests
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together