Copy them from QRegExp docs in qt5compat to
doc/global/includes/corelib/port-from-qregexp.qdocinc, so that the
porting docs can be included from both Qt 6 porting guide and qt5compat.
Task-number: QTBUG-89702
Pick-to: 6.4 6.3 6.2
Change-Id: I616e2333f60f36e4851398479939fd062016748d
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>