docs: updated http2 documentation

following #1474 (5b86fd3)

Author: lamweili

docs/index.md

@@ -95,7 +95,7 @@ The HTTP method defaults to __GET__, so if you wish, the following is valid:
 
 ## Using HTTP/2
 
-To make a request using HTTP/2 protocol only (with no HTTP/1.x fallback), use the `.http2()` method. Currently we do not have auto-detection of HTTP/2-capable servers. This feature is experimental.
+To make a request using HTTP/2 protocol only (with no HTTP/1.x fallback), use the `.http2()` method.
 
 ```javascript
     const request = require('superagent');

docs/zh_CN/index.html

@@ -73,6 +73,13 @@ <h2 id="基本请求">基本请求</h2>
 
  });
 </code></pre>
+<h2 id="使用-http-2">使用 HTTP/2</h2>
+<p>要使用 HTTP/2 协议（没有 HTTP/1.x 后备），请使用 <code>.http2()</code> 方法。</p>
+<pre><code class="language-javascript">    const request = require(&#39;superagent&#39;);
+    const res = await request
+      .get(&#39;https://example.com/h2&#39;)
+      .http2();
+</code></pre>
 <h2 id="设置请求头字段">设置请求头字段</h2>
 <p>设置请求头字段很简单，调用 <code>.set()</code> 时传入字段名称和值：</p>
 <pre><code> request

docs/zh_CN/index.md

@@ -77,6 +77,15 @@ HTTP 方法默认为 __GET__，因此如果您愿意，以下代码是有效的
 
      });
 
+## 使用 HTTP/2
+
+要使用 HTTP/2 协议（没有 HTTP/1.x 后备），请使用 `.http2()` 方法。
+
+    const request = require('superagent');
+    const res = await request
+      .get('https://example.com/h2')
+      .http2();
+
 ## 设置请求头字段
 
 设置请求头字段很简单，调用 `.set()` 时传入字段名称和值：

index.html

@@ -73,6 +73,13 @@ <h2 id="request-basics">Request basics</h2>
 
      });
 </code></pre>
+<h2 id="using-http/2">Using HTTP/2</h2>
+<p>To make a request using HTTP/2 protocol only (with no HTTP/1.x fallback), use the <code>.http2()</code> method.</p>
+<pre><code class="language-javascript">    const request = require(&#39;superagent&#39;);
+    const res = await request
+      .get(&#39;https://example.com/h2&#39;)
+      .http2();
+</code></pre>
 <h2 id="setting-header-fields">Setting header fields</h2>
 <p>Setting header fields is simple, invoke <code>.set()</code> with a field name and value:</p>
 <pre><code class="language-javascript">     request