Add note about _.runInContext to _.mixin docs. [ci skip]

lodash.src.js

@@ -11065,6 +11065,9 @@
      * destination object. If `object` is a function then methods are added to
      * its prototype as well.
      *
+     * **Note:** Use `_.runInContext` to create a pristine `lodash` function
+     * for mixins to avoid conflicts caused by modifying the original.
+     *
      * @static
      * @memberOf _
      * @category Utility
@@ -11082,7 +11085,7 @@
      *   });
      * }
      *
-     * // use `_.runInContext` to avoid potential conflicts (esp. in Node.js)
+     * // use `_.runInContext` to avoid conflicts (esp. in Node.js)
      * var _ = require('lodash').runInContext();
      *
      * _.mixin({ 'vowels': vowels });