Mention dynamic hostname lookup in nghttpx how-to

doc/sources/nghttpx-howto.rst

@@ -356,6 +356,21 @@ TLS can be enabed per pattern basis:
 In the above case, connection to serv1 will be encrypted by TLS.  On
 the other hand, connection to serv2 will not be encrypted by TLS.
 
+Dynamic hostname lookup
+-----------------------
+
+By default, nghttpx performs backend hostname lookup at start up, or
+configuration reload, and keeps using them in its entire session.  To
+make nghttpx perform hostname lookup dynamically, use ``dns``
+parameter in :option:`--backend` option, like so:
+
+.. code-block:: text
+
+   backend=foo.example.com;;dns
+
+nghttpx will cache resolved addresses for certain period of time.  To
+change this cache period, use :option:`--dns-cache-cache-timeout`.
+
 Migration from nghttpx v1.8.0 or earlier
 ----------------------------------------