Rename instance method param to instanceOptions (vuejs#1026)

* Rename instance method param to instanceOptions

It's unclear that the options parameter for the plugin has no relation to the options parameter for the instance method. Using the single name "options" is confusing.

* Update instanceOptions to methodOptions in plugins.md

Author: james-brndwgn

src/v2/guide/plugins.md

@@ -43,7 +43,7 @@ MyPlugin.install = function (Vue, options) {
     ...
   })
   // 4. インスタンスメソッドを追加
-  Vue.prototype.$myMethod = function (options) {
+  Vue.prototype.$myMethod = function (methodOptions) {
     // 何らかのロジック ...
   }
 }