Add a qdoc macro for the (non-member) swap() documentation blurb

Add an \adlswap macro (or command) capable of producing the usual member-swap() documentation blurb: https://doc.qt.io/qt-6/qstring.html#swap also for non-members, which we usually neglected to document, but some classes do (e.g. https://doc.qt.io/qt-6/qscopedpointer-obsolete.html#swap (in case https://codereview.qt-project.org/c/qt/qtbase/+/596302 has merged: that's the binary (non-member) swap overload at the time of writing).