docs(mod): should return undefined if entity is not modifier

Author: blond

README.md

@@ -37,7 +37,7 @@ const entityName = new BemEntityName({ block: 'button', elem: 'text' });
 
 entityName.block; // button
 entityName.elem;  // text
-entityName.mod;   // {}
+entityName.mod;   // undefined
 
 entityName.id;   // button__elem
 entityName.type; // elem
@@ -143,13 +143,16 @@ name.elem; // text
 
 The modifier of this entity.
 
-If entity is not modifier then returns empty object.
+**Important:** If entity is not a modifier then returns `undefined`.
 
 ```js
 const BemEntityName = require('bem-entity-name');
-const name = new BemEntityName({ block: 'button', mod: 'disabled' });
 
-name.mod; // { name: 'disabled', val: true }
+const blockName = new BemEntityName({ block: 'button' });
+const modName = new BemEntityName({ block: 'button', mod: 'disabled' });
+
+modName.mod;   // { name: 'disabled', val: true }
+blockName.mod; // undefined
 ```
 
 ### id

index.js

@@ -76,13 +76,16 @@ module.exports = class BemEntityName {
     /**
      * Returns the modifier of this entity.
      *
-     * If entity is not modifier then returns empty object.
+     * Important: If entity is not a modifier then returns `undefined`.
      *
      * @example
      * const BemEntityName = require('bem-entity-name');
-     * const name = new BemEntityName({ block: 'button', mod: 'disabled' });
      *
-     * console.log(name.mod); // { name: 'disabled', val: true }
+     * const blockName = new BemEntityName({ block: 'button' });
+     * const modName = new BemEntityName({ block: 'button', mod: 'disabled' });
+     *
+     * console.log(modName.mod);   // { name: 'disabled', val: true }
+     * console.log(blockName.mod); // undefined
      *
      * @returns {{mod: string, val: *}} entity modifier.
      */