Add missing QHttpHeaders::isEmpty() documentation

Change-Id: Ib0d19cabc09d96b3920ade3db4d46b0bb0a6decb
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
Reviewed-by: Juha Vuolle <juha.vuolle@qt.io>
(cherry picked from commit 118ee59d8c30fd6c8609f003b694fb843559573f)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/network/access/qhttpheaders.cpp

@@ -1059,6 +1059,14 @@ void QHttpHeaders::reserve(qsizetype size)
     d->headers.reserve(size);
 }
 
+/*!
+    \fn bool QHttpHeaders::isEmpty() const noexcept
+
+    Returns \c true if the headers have size 0; otherwise returns \c false.
+
+    \sa size()
+*/
+
 /*!
     Returns a header name corresponding to the provided \a name as a view.
 */