Update all hierarchy charts in the docs

Added notes on how the abstracts are represented, ensured all the class
hierarchies were up to date, and changed the orientation so the classes
are actually readable in the big chart.

docs/images/spi_device_hierarchy.dot

@@ -1,5 +1,5 @@
 digraph classes {
-    graph [rankdir=BT];
+    graph [rankdir=RL];
     node [shape=rect, style=filled, fontname=Sans, fontsize=10];
     edge [];