doc update about ifelse op in cifar SC

doc/cifarSC2011/advanced_theano.txt

@@ -211,14 +211,13 @@ Conditions
 ----------
 **IfElse**
 
-- build condition over symbolic variables
-- IfElse Op takes a boolean condition and two variables to compute as input
-- while switch evaluate both 'output' variables, IfElse is lazy and only
-  evaluate one variable respect to the condition
+- Build condition over symbolic variables.
+- IfElse Op takes a boolean condition and two variables to compute as input.
+- While Switch Op evaluates both 'output' variables, IfElse Op is lazy and only
+  evaluates one variable respect to the condition.
 
 **IfElse Example: Comparison with Switch**
 
-
 .. code-block:: python
 
   from theano import tensor as T
@@ -256,13 +255,17 @@ Conditions
 IfElse Op spend less time (about an half) than Switch since it computes only
 one variable instead of both.
 
-
 >>> python ifelse_switch.py
 time spent evaluating both values 0.6700 sec
 time spent evaluating one value 0.3500 sec
 
 Note that IfElse condition is a boolean while Switch condition is a tensor, so
-the comparison might be unfair.
+Switch is more general.
+
+It is actually important to use  ``linker='vm'`` or ``linker='cvm'``,
+otherwise IfElse will compute both variables and take the same computation
+time as the Switch Op. The linker is not currently set by default to 'cvm' but
+it will in a near future.
 
 Loops
 -----