added doc for sparse dot

doc/library/sparse/index.txt

@@ -43,6 +43,10 @@ grad?
       constant.  This function is called "structured_dot"
     - theano.sparse.structured_dot and its grad (structured_dot_grad)
     - theano.dot call it.
+    - dot(sparse, dense) and dot(dense, sparse) through transposition
+
+- Dot
+    - performs the true dot
     - dot(sparse, dense), dot(dense, sparse), dot(sparse, sparse)
 
 Subtensor selection (aka. square-bracket notation, aka indexing) is not implemented, but the