doc the new config value

doc/library/config.txt

@@ -389,13 +389,15 @@ import theano and print the config variable, as in:
 
 .. attribute:: on_opt_error
 
-    String value: 'warn', 'raise' or 'pdb'
+    String value: 'warn', 'raise', 'pdb' or 'ignore'
 
     Default: 'warn'
 
     When a crash occurs while trying to apply some optimization, either warn
     the user and skip this optimization ('warn'), raise the exception
-    ('raise'), or fall into the pdb debugger ('pdb').
+    ('raise'), fall into the pdb debugger ('pdb') or ignore it ('ignore').
+    We suggest to never use 'ignore' except in tests. If you got such warning,
+    email theano-dev about it so that we fix it.
 
 
 .. attribute:: assert_no_cpu_op