Create qdoc macros for C++ class docs 1.4: member-swap(), \internal

We have some patterns for how to document certain functions, but we
also vary the sentences a lot, and you have to look up one
documentation piece and copy it, essentially. If we ever want to
change them, we end up with shotgun surgery.

So apply DRY to the documentation and start a collection of macros to
help with repetitive C++ class documentation tasks.

The first macro is for member-swap(), and this fourth patch is for
documentation that used to be \internal. But the classes are public,
so don't hide the swap() member functions, document them.

Task-number: QTBUG-129573
Pick-to: 6.7 6.5 6.2
Change-Id: Id81b5ace05a7ccaf2a2194ebd56af10e8a98d7be
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
(cherry picked from commit 5a9bd64)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/gui/image/qpixmapcache.cpp

@@ -144,8 +144,8 @@ bool QPixmapCache::Key::operator ==(const Key &key) const
 
 /*!
     \fn void QPixmapCache::Key::swap(Key &)
-    \internal
     \since 5.6
+    \memberswap{key}
 */
 
 /*!

src/opengl/qopenglpixeltransferoptions.cpp

@@ -21,7 +21,7 @@ QT_BEGIN_NAMESPACE
 
 /*!
  * \fn void QOpenGLPixelTransferOptions::swap(QOpenGLPixelTransferOptions &other)
- * \internal
+ * \memberswap{options}
  */
 
 class QOpenGLPixelTransferOptionsData : public QSharedData