small fix following code review

doc/library/config.txt

@@ -579,8 +579,7 @@ import theano and print the config variable, as in:
   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.
+  Useful to disable it completely when the auto detection crashes.
 
 .. attribute:: config.dnn.conv.workmem
 

doc/library/sandbox/cuda/dnn.txt

@@ -38,10 +38,11 @@ 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.
 
-The recently added Theano flag :attr:`dnn.enabled <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``.
+The recently added Theano flag :attr:`dnn.enabled
+<config.dnn.enabled>` allows to change the default behavior to force
+it or disable it. Older Theano version do not support this flag. To
+get an error when CuDNN can not be sued with them, use this flag:
+``optimizer_including=cudnn``.
 
 .. note::
 

theano/configdefaults.py

@@ -330,7 +330,10 @@ AddConfigVar('dnn.library_path',
              StrParam(default_dnn_path('lib64')))
 
 AddConfigVar('dnn.enabled',
-             "If False, disable cudnn",
+             "'auto', use CuDNN is available, but silently fall back"
+             " to not using it if not present."
+             " If True and CuDNN can not be used, raise an error."
+             " If False, disable cudnn",
              StrParam("auto", "True", "False"),
              in_c_key=False)