add example of optional property (#176)

Fixes: #172

Author: zhengzongyi

content/en/tags-property.md

@@ -50,3 +50,17 @@ var config = {
 };
 ```
 {% endexample %}
+
+The following example shows how to indicate that a property is optional.
+
+ {% example "A type definition with required and optional property" %}
+
+ ```
+/**
+ * User type definition
+ * @typedef {Object} User
+ * @property {string} email
+ * @property {string} [nickName]
+ */
+ ```
+ {% endexample %}

tags-property.html

@@ -69,8 +69,16 @@ <h2 id="examples">Examples</h2>
         }
     }
 };
-</code></pre>
-    </figure>
+</code></pre></figure>
+    <p>The following example shows how to indicate that a property is optional.</p>
+    <figure>
+      <figcaption>A type definition with required and optional property</figcaption><pre class="prettyprint"><code>/**
+ * User type definition
+ * @typedef {Object} User
+ * @property {string} email
+ * @property {string} [nickName]
+ */
+</code></pre> </figure>
     <h2 id="related-links">Related Links</h2>
     <ul>
       <li>
@@ -83,8 +91,7 @@ <h2 id="related-links">Related Links</h2>
   </article>
   <footer>
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-    </a>
+      <img alt="Creative Commons License" class="license-badge" src="images/cc-by-sa.svg" width="80" height="15" /></a>
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