Added doc about the different linker.

doc/tutorial/modes.txt

@@ -36,6 +36,31 @@ DEBUG_MODE        ``compile.debugmode.DebugMode()``
 PROFILE_MODE      ``compile.profilemode.ProfileMode()``                           C implementations where available, all available graph transformations, print profile information.
 ================= =============================================================== ===============================================================================
 
+Mode detail
+===========
+
+A mode is composed of 2 thinks: an optimizer and a linker. Some mode like PROFILE_MODE and DEBUG_MODE add logic around the optimizer and linker.PROFILE_MODE and DEBUG_MODE use there own linker.
+
+You can select witch linker to use with the Theano flag linker. Here is a table to compare the different linker.
+
+=============  =========  =================  =========  ===
+linker         gc [#gc]_  Raise error by op  Overhead   Definition
+=============  =========  =================  =========  ===
+c|py [#cpy1]_  yes        yes                "+++"      Try c code. If none exist for an op, use python
+c|py_nogc      no         yes                "++"       As c|py but don't gc
+c              no         yes                "+"        Use only c code(if none available for an op, raise an error)
+py             yes        yes                "+++"      Use only python code
+c&py [#cpy2]_  no         yes                "+++++"     Use c and python code
+ProfileMode    no         no                 "++++"     Compute some extra profiling info
+DebugMode      no         yes                VERY HIGH  Make many check on what Theano compute.
+=============  =========  =================  =========  ===
+
+
+.. [#gc] Garbadge collection of intermediate result during computation.
+         Otherwise they the memory space is keep between Theano function call
+         To don't reallocate memory and lower the overhead(make it faster...)
+.. [#cpy1] default
+.. [#cpy2] Deprecated
 
 
 .. _using_debugmode: