[docs] QMap: fix missing toStdMap()&& overload docs

Amends 14090760a8.

I tried to find a linkable definition of valid-but-unspecified on
en.cppreference.com, but failed, so I'm using partially-formed like
everywhere else in Qt docs.

Ideally, we'd have the discussion of partially-formed (and
valid-but-unspecified) in some extra page and simply link to
it. Created QTBUG-106251 to track the issue.

Pick-to: 6.4 6.3 6.2
Change-Id: I04c60cf903b2617c89467d1d040d5aebb7eccd53
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qmap.qdoc

@@ -202,6 +202,15 @@
     Returns an STL map equivalent to this QMap.
 */
 
+/*! \fn template <class Key, class T> std::map<Key, T> QMap<Key, T>::toStdMap() &&
+
+    \overload
+    \since 6.0
+
+    \note Calling this function will leave this QMap in the partially-formed state, in which
+    the only valid operations are destruction or assignment of a new value.
+*/
+
 /*! \fn template <class Key, class T> bool QMap<Key, T>::operator==(const QMap<Key, T> &lhs, const QMap<Key, T> &rhs)
 
     Returns \c true if \a lhs is equal to \a rhs; otherwise returns