Recommend return type annotation (#1239)

* Recommend return type annotation

* make explanation clear, thank @DanielRosenwasser

Author: HerringtonDarkholme

src/v2/guide/typescript.md

@@ -141,3 +141,39 @@ var vm = new Vue({
   myOption: 'Hello'
 })
 ```
+
+## Annotating Return Types
+
+Because of the circular nature of Vue's declaration files, TypeScript may have difficulties inferring the types of certain methods.
+For this reason, you may need to annotate the return type on methods like `render` and those in `computed`.
+
+```ts
+import Vue, { VNode } from 'vue'
+
+const Component = Vue.extend({
+  data() {
+    return {
+      msg: 'Hello'
+    }
+  },
+  methods: {
+    // need annotation due to `this` in return type
+    greet(): string {
+      return this.msg + ' world'
+    }
+  },
+  computed: {
+    // need annotation
+    greeting(): string {
+      return this.greet() + '!'
+    }
+  },
+  // `createElement` is inferred, but `render` needs return type
+  render(createElement): VNode {
+    return createElement('div', this.greeting)
+  }
+})
+```
+
+If you find type inference or member completion isn't working, annotating certain methods may help address these problems.
+Using the `--noImplicitAny` option will help find many of these unannotated methods.