Add contextual help for using data from show page

[hackartisan]

This can take the form of question mark icons with a tooltip next to each download type, informing users where they might use that data and what kind of uses it might have.

Can you draft a few of these sentences before we move this over to ready, @eliotjordan ?

[tpendragon]

We should re-test the usefulness of the exports after upgrading Geoblacklight, which changes this pane.