Document how to import tooltip directive

What does this MR do?

Documents how to import tooltip directive by explaining that you need to import and register GlTooltipDirective.

Screenshot

Before	After

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Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
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• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
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  • GitLab: mr_url
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• Added the ~"component:*" label(s) if applicable.

Closes #702 (closed)