insufficient explanation of Qt wayland environment variable usage

https://doc.qt.io/qt-6/qtwaylandcompositor-index.html#environment-variables-and-command-line-arguments

Should remove the unused environment variables such as

• QT_WAYLAND_SERVER_BUFFER_INTEGRATION

and for QT_WAYLAND_HARDWARE_INTEGRATION as it is the same as QT_WAYLAND_CLIENT_BUFFER_INTEGRATION.
https://github.com/qt/qtwayland/blob/dev/src/compositor/compositor_api/qwaylandcompositor.cpp#L395

so they should be merged into one. if there are some reason like backwards compatibility, we need mention it in the documentation these two are the same

Regarding QT_WAYLAND_CLIENT_BUFFER_INTEGRATION, should explain what the possible value is and what it is of each of them. Based on https://github.com/qt/qtwayland/tree/dev/src/plugins/hardwareintegration/client there are so many implementations.