Better doc for diag().

theano/sandbox/linalg/ops.py

@@ -449,7 +449,10 @@ alloc_diag = AllocDiag()
 def diag(x):
     """Numpy-compatibility method
     
-    For vector `x`, return a zero matrix except for `x` as diagonal.
+    If `x` is a matrix, return its diagonal.
+    If `x` is a vector return a matrix with it as its diagonal.
+
+    * This method does not support the `k` argument that numpy supports.
     """
     xx = as_tensor_variable(x)
     if xx.type.ndim == 1: