QNetworkRequest: Document how Qt handles 301 and 302 status codes

To make it explicit

Pick-to: 6.3 6.2 5.15
Task-number: QTBUG-92909
Change-Id: I8f8dc99628168a566158acef72ae2a5e001eef88
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

src/network/access/qnetworkrequest.cpp

@@ -410,6 +410,11 @@ QT_BEGIN_NAMESPACE
                                        for example, to ask the user whether to
                                        accept the redirect, or to decide
                                        based on some app-specific configuration.
+
+    \note When Qt handles redirects it will, for legacy and compatibility
+    reasons, issue the redirected request using GET when the server returns
+    a 301 or 302 response, regardless of the original method used, unless it was
+    HEAD.
 */
 
 /*!