Missing documentation for QSGRenderNode::RenderState

The documentation for `QSGRenderNode` mentions `QSGRenderNode::RenderState` as a parameter type in `QSGRenderNode::render` and as part of the documentation for `QSGRenderNode::RenderingFlag::DepthAwareRendering`.

Nonetheless, the class documentation for `QSGRenderNode::RenderState` is missing, resulting in a linking failure.

In `qtdeclarative/src/quick/scenegraph/coreapi/qsgrendernode.cpp` the documentation for `QSGRenderNode` methods seems to be present, but a class topic documentation block seems to be missing.

A possibly similar element is documented in QSGMaterialShader::RenderState and it might be possible to use it as a basic template.