update hovertemplate description

Author: antoinerg

src/components/fx/hovertemplate_attributes.js

@@ -19,7 +19,7 @@ module.exports = function(opts, extra) {
         for(var i = 0; i < keys.length; i++) {
             quotedKeys[i] = '`' + keys[i] + '`';
         }
-        descPart = descPart + 'Finally, this trace also supports ';
+        descPart = descPart + 'Finally, the template string has access to ';
         if(keys.length === 1) {
             descPart = 'variable ' + quotedKeys[0];
         } else {

src/traces/sankey/attributes.js

@@ -148,7 +148,8 @@ var attrs = module.exports = overrideAll({
         },
         hoverlabel: fxAttrs.hoverlabel, // needs editType override,
         hovertemplate: hovertemplateAttrs({}, {
-            keys: []
+            description: 'Variables `sourceLinks` and `targetLinks` are arrays of link objects.',
+            keys: ['value', 'label']
         }),
         description: 'The nodes of the Sankey plot.'
     },
@@ -221,7 +222,8 @@ var attrs = module.exports = overrideAll({
         },
         hoverlabel: fxAttrs.hoverlabel, // needs editType override,
         hovertemplate: hovertemplateAttrs({}, {
-            keys: []
+            description: 'Variables `source` and `target` are node objects.',
+            keys: ['value', 'label']
         }),
         description: 'The links of the Sankey plot.'
     }