Add a tooltip to "Use snapshots only" checkbox

It would be nice to eventually have an explanation of what the "Use snapshots only" checkbox does when it's checked vs unchecked. Perhaps something like a ℹ️ with a tooltip.

_Originally posted by @john-legg in https://github.com/Automattic/compare-snapshot-tool/issues/5#issuecomment-1396229248_

Start by locating the UI that renders the “Use snapshots only” checkbox and read how its checked and unchecked states are handled. Done means adding an accessible tooltip or information affordance that explains both states.

Written by the indexing model from the issue text.