make trace uid an anim:true attr + improve description

Author: etpinard

src/plots/attributes.js

@@ -74,7 +74,13 @@ module.exports = {
     uid: {
         valType: 'string',
         role: 'info',
-        editType: 'plot'
+        editType: 'plot',
+        anim: true,
+        description: [
+            'Assign an id to this trace,',
+            'Use this to provide object constancy between traces during animations',
+            'and transitions.'
+        ].join(' ')
     },
     ids: {
         valType: 'data_array',