Doc dnn.enable flag

doc/library/config.txt

@@ -567,6 +567,19 @@ import theano and print the config variable, as in:
 
     A directory with bin/, lib/, include/ folders containing cuda utilities.
 
+.. attribute:: dnn.enabled
+
+  String value: ``auto``, ``True``, ``False``
+
+  Default ``auto``
+
+  If ``auto``, auto detect if cudnn is there, if not, raise an error.
+
+  If ``True``, if we can't use cudnn, raise an error.
+
+  If ``False``, do not test if cudnn is there and don't use it.
+  Useful to disable it completly when the auto detection crash in some strange case.
+
 .. attribute:: config.dnn.conv.workmem
 
    Deprecated, use dnn.conv.algo_fwd.

doc/library/sandbox/cuda/dnn.txt

@@ -38,8 +38,10 @@ By default, Theano will detect if it can use cuDNN. If so, it will use
 it.  If not, Theano optimizations will not introduce cuDNN ops. So
 Theano will still work if the user did not introduce them manually.
 
-To get an error if Theano can not use cuDNN, use this Theano flag:
-``optimizer_including=cudnn``.
+The recently added Theano flag :attr:`allow_gc <config.dnn.enabled>`
+allow to change the default behavior to force it or disable it. You
+can also get an error with older Theano version if cuDNN can not be
+used, by using this Theano flag: ``optimizer_including=cudnn``.
 
 .. note::