QVariant/Doc: document that you must register types before op>>

But not before ::fromValue, since that registers automatically.

Take the opportunity to add the \relates to the two streaming operators
to QDataStream, so the documentation shows up *somewhere* at all.

Task-number: QTBUG-105469
Change-Id: Ic6547f8247454b47baa8fffd170eb8ffc31feb5d
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
(cherry picked from commit 15ccc7e499ac8dd1f75dfa75346d15c4b4d06324)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/kernel/qvariant.cpp

@@ -1312,9 +1312,21 @@ void QVariant::save(QDataStream &s) const
 
 /*!
     \since 4.4
+    \relates QVariant
 
     Reads a variant \a p from the stream \a s.
 
+    \note If the stream contains types that aren't the built-in ones (see \l
+    QMetaType::Type), those types must be registered using qRegisterMetaType()
+    or QMetaType::registerType() before the variant can be properly loaded. If
+    an unregistered type is found, QVariant will set the corrupt flag in the
+    stream, stop processing and print a warning. For example, for QList<int>
+    it would print the following:
+
+    \quotation
+    QVariant::load: unknown user type with name QList<int>
+    \endquotation
+
     \sa{Serializing Qt Data Types}{Format of the QDataStream operators}
 */
 QDataStream &operator>>(QDataStream &s, QVariant &p)
@@ -1325,6 +1337,7 @@ QDataStream &operator>>(QDataStream &s, QVariant &p)
 
 /*!
     Writes a variant \a p to the stream \a s.
+    \relates QVariant
 
     \sa{Serializing Qt Data Types}{Format of the QDataStream operators}
 */
@@ -2595,9 +2608,6 @@ QT_WARNING_POP
 
     \snippet code/src_corelib_kernel_qvariant.cpp 7
 
-    \note If you are working with custom types, you should use
-    the Q_DECLARE_METATYPE() macro to register your custom type.
-
     \sa setValue(), value()
 */