Clarity on Historic / Current button

[james-rae]

Describe the bug

I'm not sure I'd know what these buttons mean or do. Maybe add a line in the "How to use the map" section? Alternately there is enough room for Current Data and Historic Data in English. Maybe that's good enough? Don't know what the French would be (see #18)

Visually you can sorta guess when you see the slider appear. Not sure if screen reader would be aware? Is something similar to RAMP's updateAlert() being used? If we don't want instructions might need that.