Documentation: have a central place to explain partially-formed-ness

Currently, all documented move special members re-explain what partially-formed means (e.g. https://doc.qt.io/qt-6/qpen.html#QPen-5, https://doc.qt.io/qt-6/qhostinfo.html#QHostInfo-2).

We should have a somewhat more verbose discussion of partially-formed-ness (also in constract to std C++'s valid-but-unspecified), so such functions can simply link to it:

\note The moved-from object is placed in the \l{partially-formed} state.