Show hint when hovering over function icon

src/components/fields/ZoomSpecField.jsx

@@ -174,10 +174,16 @@ export default class ZoomSpecField extends React.Component {
             verticalAlign: 'top',
             backgroundColor: null,
             display: 'inline-block',
+            paddingBottom: 0,
+            paddingTop: 0,
           }}
           onClick={this.makeZoomFunction.bind(this)}
         >
-          <FunctionIcon />
+          <DocLabel
+            label={<FunctionIcon />}
+            cursorTargetStyle={{ cursor: 'pointer' }}
+            doc="Turn property into a zoom function to enable a map feature to change with map's zoom level."
+          />
         </Button>
         }
         <SpecField {...this.props} style={{ width: '50%' } }/>