better doc of new print_active_device theano flag.

doc/library/config.txt

@@ -161,6 +161,14 @@ import theano and print the config variable, as in:
     Theano initialize the GPU device.  Newer version of PyCUDA
     (currently only in the trunk) don't have this restriction.
 
+.. attribute:: config.print_active_device
+
+    Bool value: either ``True`` or ``False``
+
+    Default: ``True``
+
+    Print active device at when the GPU device is initialized.
+
 .. attribute:: floatX
 
     String value: either 'float64' or 'float32'

theano/configdefaults.py

@@ -76,7 +76,7 @@ AddConfigVar('force_device',
         in_c_key=False)
 
 AddConfigVar('print_active_device',
-        "Print active device at startup",
+        "Print active device at when the GPU device is initialized.",
         BoolParam(True, allow_override=False),
         in_c_key=False)