Add screen and context to docs.

src/gallium/docs/source/context.rst

@@ -0,0 +1,10 @@
+Context
+=======
+
+The context object represents the purest, most directly accessible, abilities
+of the device's 3D rendering pipeline.
+
+Methods
+-------
+
+XXX

src/gallium/docs/source/index.rst

@@ -13,6 +13,8 @@ Contents:
 
    intro
    tgsi
+   screen
+   context
    cso
 
 Indices and tables

src/gallium/docs/source/screen.rst

@@ -0,0 +1,9 @@
+Screen
+======
+
+A screen is an object representing the context-independent part of a device.
+
+Methods
+-------
+
+XXX