Document QHostAddress::swap

Change-Id: I0f47ffeb1a7dbda7dadbd78b2ea04167c42a503d
Reviewed-by: Jesus Fernandez <Jesus.Fernandez@qt.io>
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

src/network/kernel/qhostaddress.cpp

@@ -605,6 +605,14 @@ QHostAddress &QHostAddress::operator=(SpecialAddress address)
     return *this;
 }
 
+/*!
+    \fn void QHostAddress::swap(QHostAddress &other)
+    \since 5.6
+
+    Swaps this host address with \a other. This operation is very fast
+    and never fails.
+*/
+
 /*!
     \fn bool QHostAddress::operator!=(const QHostAddress &other) const
     \since 4.2