Add req.host property documentation to 5.x API

_includes/api/en/5x/menu.md

@@ -80,6 +80,8 @@
             </li>
             <li><a href="#req.fresh">req.fresh</a>
             </li>
+            <li><a href="#req.host">req.host</a>
+            </li>
             <li><a href="#req.hostname">req.hostname</a>
             </li>
             <li><a href="#req.ip">req.ip</a>

_includes/api/en/5x/req-host.md

@@ -0,0 +1,22 @@
+<h3 id='req.host'>req.host</h3>
+
+Contains the host derived from the `Host` HTTP header.
+
+When the [`trust proxy` setting](api.html#app.settings.table)
+does not evaluate to `false`, this property will instead get the value
+from the `X-Forwarded-Host` header field. This header can be set by
+the client or by the proxy.
+
+If there is more than one `X-Forwarded-Host` header in the request, the
+value of the first header is used. This includes a single header with
+comma-separated values, in which the first value is used.
+
+```js
+// Host: "example.com:3000"
+console.dir(req.host)
+// => 'example.com:3000'
+
+// Host: "[::1]:3000"
+console.dir(req.host)
+// => '[::1]:3000'
+```

_includes/api/en/5x/req.md

@@ -56,6 +56,10 @@ or [pez](https://www.npmjs.com/package/pez).
   {% include api/en/5x/req-fresh.md %}
 </section>
 
+<section markdown="1">
+  {% include api/en/5x/req-host.md %}
+</section>
+
 <section markdown="1">
   {% include api/en/5x/req-hostname.md %}
 </section>