Updated comment following code review.

doc/library/config.txt

@@ -307,9 +307,6 @@ import theano and print the config variable, as in:
     Default: $CUDA_ROOT or failing that, "/usr/local/cuda"
 
     A directory with bin/, lib/, include/ folders containing cuda utilities.
-    If set to an empty string (e.g. with ``root=`` in the ``[cuda]`` section
-    of your .theanorc file), then it will disable warnings about cuda not
-    being found.
 
 .. attribute:: config.gcc.cxxflags