Small docstring update.

theano/compile/function.py

@@ -143,10 +143,10 @@ def function(inputs, outputs=None, mode=None, updates=None, givens=None,
                with all the Apply nodes in the graph in the correct order.
                the cvm is a linker that replaces this python loop with a c
                loop to avoid continuously changing between python and c.
-               (It's currently a matter of disagreement as to whether there
-               is a cost of changing between
-               python and C, or whether the cvm is faster only because C code
-               is faster than python code)
+               The CVM is faster for 2 reasons:
+                 1) It's internal logic in C, so no Python interpreter overhead.
+                 2) It makes native calls from the VM logic into thunks that
+                 have been compiled using the CLinker.
                the vm is a linker that was developed to prototype the cvm. it
         was easier to develop the vm in python then translate it to c instead
         of just writing it in c from scratch

theano/gof/graph.py

@@ -220,13 +220,13 @@ class Variable(utils.object2):
     - `Constant` (a subclass) which adds a default and un-replaceable :literal:`value`, and
       requires that owner is None
 
-    - `TensorVariable`
+    - `TensorVariable` subclass of Variable that represent numpy.ndarray object
 
-    - `SharedTensorVariable`
+    - `SharedTensorVariable` Shared version of TensorVariable
 
-    - `SparseVariable`
+    - `SparseVariable` subclass of Variable that represent scipy.sparse.{csc,csr}_matrix object
 
-    - `CudaVariable`
+    - `CudaNdarrayVariable` subclass of Variable that represent our object on the GPU that is a subset of numpy.ndarray
 
     - `RandomVariable`