Add explanation of stickiness

frontend/src/scenes/trends/ShownAsFilter.js

@@ -16,6 +16,7 @@ export function ShownAsFilter({ shown_as, onChange }) {
                     onChange={item => onChange(item.value)}
                 />
             </div>
+            {shown_as == 'Stickiness' && <small><i>Stickiness shows you how many days users performed an action within the timeframe. If a user did an action on Monday and came back and did it twice on Friday, it would be listed as "2 days" in the chart.</i></small>}
         </div>
     )
 }