clearColor in SceneEnvironment needs clarification wrt View3D.renderMode

The Lancelot differences in the 'view3d' tests shown in QTBUG-83364 are something that popped up a couple of times in the past: clearColor is not quite supported with the RHI-based rendering path when the View3D.renderMode is not set to Offscreen.

The reason is simple: when rendering to a texture (Offscreen) we have full control over the content because the entire render pass is under Quick3D's control. With the underlay mode this is not the case, Quick3D merely plugs in to issue QRhiCommandBuffer calls on a command buffer that already has the render pass started by the Qt Quick scenegraph (i.e. beginPass() has already been called). This makes it impossible to manage clearing of the color and other buffers - it the scenegraph's responsibility and it happens before Quick3D gets a chance to do anything.

We already mitigated this in the examples, but it is not yet documented and the Lancelot cases have not been updated.