fix gh-2929

doc/tutorial/modes.txt

@@ -137,7 +137,7 @@ is controlled by the value of the ``mode`` parameter.
 
 Theano defines the following modes by name:
 
-- ``'FAST_COMPILE'``: Apply just a few graph optimizations and only use Python implementations.
+- ``'FAST_COMPILE'``: Apply just a few graph optimizations and only use Python implementations. So GPU is disabled.
 - ``'FAST_RUN'``: Apply all optimizations and use C implementations where possible.
 - ``'DebugMode``: Verify the correctness of all optimizations, and compare C and Python 
     implementations. This mode can take much longer than the other modes, but can identify

doc/tutorial/using_gpu.txt

@@ -783,5 +783,9 @@ Modify and execute to support *stride* (i.e. to avoid constraining the input to
 Note
 ----
 
-See :ref:`example_other_random` to know how to handle random numbers
-on the GPU.
+* See :ref:`example_other_random` to know how to handle random numbers
+  on the GPU.
+
+* The mode `FAST_COMPILE` disable c code, so also disable the GPU. You
+  can use the Theano flag optimizer='fast_compile' to speed up
+  compilatino and keeping the GPU.