Add section on PushOutSeqScan

doc/developer/scan.txt

@@ -206,6 +206,18 @@ it to the outer function to be executed only once, before the Scan Op,
 reduces the amount of computation that needs to be performed.
 
 
+PushOutSeqScan
+--------------
+
+This optimization resembles PushOutNonSeqScan but it tries to push, out of
+the inner function, the computation that only relies on sequence and
+non-sequence inputs. The idea behing this optimization is that, when it is
+possible to do so, it is generally more computationnaly efficient to perform
+a single operation on a large tensor rather then perform that same operation
+many times on many smaller tensors. However, this optimization can lead to
+increased memory usage.  
+
+