Small doc about about fastmath and nan.

doc/library/config.txt

@@ -79,8 +79,8 @@ Environment Variables
     (e.g. ``config.device``, ``config.mode``) should be defined in the
     ``[global]`` section.
     Attributes from a subsection of ``config`` (e.g. ``config.lib.cnmem``,
-    ``dnn.conv.algo_fwd``) should be defined in their corresponding section
-    (e.g. ``[nvcc]``, ``[dnn.conv]``).
+    ``config.dnn.conv.algo_fwd``) should be defined in their corresponding
+    section (e.g. ``[nvcc]``, ``[dnn.conv]``).
 
     Multiple configuration files can be specified by separating them with ':'
     characters (as in $PATH).  Multiple configuration files will be merged,

doc/tutorial/nan_tutorial.txt

@@ -69,3 +69,10 @@ In the most difficult situations, you may go through the above steps and find
 nothing wrong. If the above methods fail to uncover the cause, there is a good
 chance that something is wrong with your algorithm. Go back to the mathematics
 and find out if everything is derived correctly.
+
+
+Cuda Specific Option
+--------------------
+
+The Theano flags ``nvcc.fastmath=True``, can genarate NaN. Don't set
+this flag while debugging nan.