Added watching a nested property to API reference

vuejs/docs@2106042

Author: naokie

src/api/options-data.md

@@ -190,11 +190,12 @@
         c: {
           d: 4
         },
-        e: 'test',
-        f: 5
+        e: 5,
+        f: 6
       }
     },
     watch: {
+      // watching top-level property
       a(val, oldVal) {
         console.log(`new: ${val}, old: ${oldVal}`)
       },
@@ -207,6 +208,10 @@
         },
         deep: true
       },
+      // watching a single nested property:
+      'c.d': function (val, oldVal) {
+        // do something
+      },
       // the callback will be called immediately after the start of the observation
       e: {
         handler(val, oldVal) {