It works since 6.6 regardless, but the help text had no mentioning of it
at all.
Also use the nicer "opengl" instead of "gl" (both are accepted by
QSG_RHI_BACKEND anyway)
Pick-to: 6.7 6.6
Change-Id: Ibf6de9411d8641d244b8750fd7a015781a9297b1
Reviewed-by: Andy Nichols <andy.nichols@qt.io>