Add watch usages (vuejs#1356)

* Add watch usages!

Add `watch` usages!

* Update index.md

* tweaks to watch api examples

Author: wangyi7099

src/v2/api/index.md

@@ -592,7 +592,7 @@ type: api
 
 ### watch
 
-- **Type:** `{ [key: string]: string | Function | Object }`
+- **Type:** `{ [key: string]: string | Function | Object | Array}`
 
 - **Details:**
 
@@ -606,7 +606,12 @@ type: api
       a: 1,
       b: 2,
       c: 3,
-      d: 4
+      d: 4,
+      e: {
+        f: {
+          g: 5
+        }
+      }
     },
     watch: {
       a: function (val, oldVal) {
@@ -623,7 +628,13 @@ type: api
       d: {
         handler: function (val, oldVal) { /* ... */ },
         immediate: true
-      }
+      },
+      e: [
+        function handle1 (val, oldVal) { /* ... */ },
+        function handle2 (val, oldVal) { /* ... */ }
+      ],
+      // watch vm.e.f's value: {g: 5}
+      'e.f': function (val, oldVal) { /* ... */ }
     }
   })
   vm.a = 2 // => new: 2, old: 1