Fix qdoc warning from documentation of internal API

QXmlString is an internal class, so qdoc cannot tie the member function
documentation to anything. Since "swap" is rather self-explanatory anyway,
we can just remove the (somewhat oddly placed) documentation.

Change-Id: I3d47219a628ba3d3e5ecd0c825eeff71908d07e9
Reviewed-by: Paul Wicking <paul.wicking@qt.io>

src/corelib/serialization/qxmlstream.cpp

@@ -2530,13 +2530,6 @@ QXmlStreamEntityDeclaration::QXmlStreamEntityDeclaration()
 {
 }
 
-/*! \fn QXmlString::swap(QXmlString &other)
-    \since 6.0
-
-    Swaps this string reference's contents with \a other.
-    This function is very fast and never fails.
-*/
-
 /*! \fn QStringView QXmlStreamEntityDeclaration::name() const
 
 Returns the entity name.