added exception_verbosity to doc/library/config.txt

doc/library/config.txt

@@ -475,3 +475,17 @@ import theano and print the config variable, as in:
     ``'warn'`` will result in a UserWarning being raised when some Op inputs
     do not contain an appropriate test value. ``'raise'`` will instead raise
     an Exception when a problem is encountered during this debugging phase.
+    
+.. attribute:: config.exception_verbosity
+
+    String Value: ``'low'``, ``'high'``.
+
+    Default: ``'low'``
+  
+    If ``'low'``, the text of exceptions will generally refer to apply nodes
+    with short names such as ``'Elemwise{add_no_inplace}'``. If ``'high'``,
+    some exceptions will also refer to apply nodes with long descriptions like:
+
+	A. Elemwise{add_no_inplace}
+	      B. log_likelihood_v_given_h
+	      C. log_likelihood_h