Adds constant description for units

Makes `km/h` constant with distances

public/app/core/utils/kbn.ts

@@ -989,17 +989,17 @@ kbn.getUnitFormats = function() {
     {
       text: 'velocity',
       submenu: [
-        { text: 'm/s', value: 'velocityms' },
-        { text: 'km/h', value: 'velocitykmh' },
-        { text: 'mph', value: 'velocitymph' },
+        { text: 'metres/second (m/s)', value: 'velocityms' },
+        { text: 'kilometers/hour (km/h)', value: 'velocitykmh' },
+        { text: 'miles/hour (mph)', value: 'velocitymph' },
         { text: 'knot (kn)', value: 'velocityknot' },
       ],
     },
     {
       text: 'volume',
       submenu: [
-        { text: 'millilitre', value: 'mlitre' },
-        { text: 'litre', value: 'litre' },
+        { text: 'millilitre (mL)', value: 'mlitre' },
+        { text: 'litre (L)', value: 'litre' },
         { text: 'cubic metre', value: 'm3' },
         { text: 'Normal cubic metre', value: 'Nm3' },
         { text: 'cubic decimetre', value: 'dm3' },