Add the new flag in documentation.

doc/library/config.txt

@@ -671,6 +671,14 @@ import theano and print the config variable, as in:
 
     A directory with bin/, lib/, include/ folders containing cuda utilities.
 
+.. attribute:: config.cuda.enabled
+
+    Bool value: either ``True`` of ``False``
+
+    Default: ``True``
+
+    If set to `False`, C code in old backend is not compiled.
+
 .. attribute:: config.dnn.enabled
 
     String value: ``'auto'``, ``'True'``, ``'False'``

theano/configdefaults.py

@@ -199,7 +199,6 @@ AddConfigVar(
     StrParam(default_cuda_root),
     in_c_key=False)
 
-# To be used as an internal config or for developers (i.e. not to be put in documentation)
 AddConfigVar(
     'cuda.enabled',
     'If false, C code in old backend is not compiled.',