Finished doc.

doc/library/scan.txt

@@ -529,8 +529,8 @@ As a rule, scan always expects the condition to be the last thing returned
 by the inner function, otherwise an error will be raised.
 
 
-Reduce Scan's memory usage
---------------------------
+Reducing Scan's memory usage
+----------------------------
 
 This section presents the ``scan_with_checkpoints`` function. In short, this
 function reduces the memory usage of scan (at the cost of more computation
@@ -539,14 +539,16 @@ and recomputing them when computing the gradients. This function is therefore
 only useful if you need to compute the gradient of the ouptut of scan with
 respect to its inputs, and shouldn't be used otherwise.
 
-Before going more into the details, here are a few current limitations:
+Before going more into the details, here are its current limitations:
 
 * It only works in the case where only the output of the last time step is
   needed, like when computing ``A**k`` or in an `encoder-decoder` setup.
 * It only accepts sequences of the same length.
 * If ``n_steps`` is specified, it has the same value as the length of any
   sequences.
-* Only singly-recurrent and non-recurrent outputs are used. TODO WHAT DOES IT MEANS?
+* It is signly-recurrent, meaning that only the previous time step can be used
+  to compute the current one (ie ``h[t]`` can only depend on ``h[t-1]``). In
+  other words, ``taps`` can not be used in ``sequences`` and ``outputs_info``.
 
 Often, in order to be able to compute the gradients through scan operations,
 Theano needs to keep in memory some intermediate computations of scan. This