Improve documentation on QSGMaterial

I suggest to update documentation on QSGMaterial. In particular append
to Detailed description a link to Qt Quick Scene Graph Renderer document.
Also link to 'Qt Quick Scene Graph Renderer -> Batch Roots
-> Transform Nodes' should be added to QSGMaterial::RequiresFullMatrix flag
documentation with description (or even just link to Performance chapter) 'copied' from Performance
chapter - Settings this flag prevents all batching.

The motive for this suggestion is that most developers (me to)
find the sample 'Scene Graph - Graph' and try to hack it for
their needs - custom geometry and shaders. But they may
be astonished - 'where is my triangle, its coordinates in NDC?!' -
similiar https://bugreports.qt.io/browse/QTBUG-38545 .
And then devs go to read docs on QSGMaterial, QSGMaterialShader,
QSGGeometry, QSGGeometryNode, QEtc BUT hardly to discover
the mentioned doc 'Qt Quick Scene Graph Renderer' and
comprehend what happend there actually.