Doc: document QTimeZone::swap()

Change-Id: I80d55df65546b671772ed93cb852e88ee040114a
Reviewed-by: Sze Howe Koh <szehowe.koh@gmail.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>

src/corelib/tools/qtimezone.cpp

@@ -417,6 +417,13 @@ QTimeZone::~QTimeZone()
 {
 }
 
+/*!
+    \fn QTimeZone::swap(QTimeZone &other)
+
+    Swaps this time zone instance with \a other. This function is very
+    fast and never fails.
+*/
+
 /*!
     Assignment operator, assign \a other to this.
 */