Add a note to QFormDataPartBuilder::setBodyDevice

If the body is sequential, QNAM::post should be called after body has
emitted finished.

As this is already mentioned in the documentation of
QHttpPart::setBodyDevice it probably won't hurt to mention it here
as well.

Task-number: QTBUG-125985
Pick-to: 6.8
Change-Id: Iaa29c45b8da41a9af893523ff140afa9a0945f95
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/network/access/qformdatabuilder.cpp

@@ -167,6 +167,10 @@ QFormDataPartBuilder &QFormDataPartBuilder::setBody(QByteArrayView data,
     ownership of \a body, i.e. the device must be closed and destroyed if
     necessary.
 
+    \note If \a body is sequential (e.g. sockets, but not files),
+    QNetworkAccessManager::post() should be called after \a body has emitted
+    finished().
+
     \sa setBody(), QHttpPart::setBodyDevice()
   */