improving comment of the error

doc/library/tensor/basic.txt

@@ -1124,7 +1124,9 @@ Theano indexing with a "mask" (incorrect approach):
    >>> t[t > 4].eval()  # an array with shape (3, 3, 3)  # doctest: +ELLIPSIS
    Traceback (most recent call last):
      ...
-   TypeError: TensorType does not support boolean mask for indexing such as tensor[x==0]. If you are indexing on purpose with an int8, please cast it to int32
+   TypeError: TensorType does not support boolean mask for indexing such as tensor[x==0].
+   Instead you can use non_zeros() such as tensor[(x == 0).nonzeros()].
+   If you are indexing on purpose with an int8, please cast it to int16.
 
 Getting a Theano result like NumPy:
 

theano/tensor/var.py

@@ -474,8 +474,10 @@ class _tensor_py_operators(object):
                    args_el.dtype == 'int8' or args_el.dtype == 'uint8':
                     raise TypeError(('TensorType does not support boolean '
                                      'mask for indexing such as tensor[x==0]. '
+                                     'Instead you can use non_zeros() such as '
+                                     'tensor[(x == 0).nonzeros()]. '
                                      'If you are indexing on purpose with an '
-                                     'int8, please cast it to int32'))
+                                     'int8, please cast it to int16.'))
             except AttributeError:
                 pass