[v10.0.x] Grafana UI: Fix behaviour regression on Tooltip component

[grafana-delivery-bot]

Backport 1cdc139 from #70682

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What is this feature?

To fix the behaviour of the Tooltip component.

Why do we need this feature?

In order to recover previous behaviour:

value	behaviour
show={undefined}	tooltip shown on hover, hidden when no longer hovering
show={true}	tooltip is always shown, never hidden
show={false}	tooltip is never shown (effectively disabled)

Who is this feature for?

Everybody

Which issue(s) does this PR fix?:

Fixes #70514 but will keep it to discuss further, for example, if would be better to have a disable prop instead.

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

[eledobleefe]

LGTM 😉