talk about Shared Variable in the doc comparaison about theano vs c.

doc/extending/theano_vs_c.txt

@@ -12,7 +12,8 @@ analogue in a statically typed language such as C:
 Theano          C
 =============== ===========================================================
 Apply           function application / function call
-Variable          function data / variable
+Variable        local function data / variable
+Shared Variable global function data / variable
 Op              operations carried out in computation / function definition
 Type            data types
 Module          class
@@ -21,19 +22,22 @@ Module          class
 For example:
 
 .. code-block:: c
+   int d = 0;
 
    int main(int a) {
        int b = 3;
        int c = f(b)
+       d = b + c;
        return g(a, c);
    }
 
 
 Based on this code snippet, we can relate ``f`` and ``g`` to Ops, ``a``,
-``b`` and ``c`` to Variables, ``g(a, c)`` and ``f(b)`` (taken as meaning
-the action of computing ``f`` or ``g`` on their respective inputs) to
+``b`` and ``c`` to Variables, ``d`` to Shared Variable, ``g(a, c)``,
+``f(b)`` and ``d = b + c`` (taken as meaning
+the action of computing ``f``, ``g`` or ``+``on their respective inputs) to
 Applies. Lastly, ``int`` could be interpreted as the Theano Type of the
-Variables ``a`` and ``b``.
+Variables ``a``, ``b``, ``c`` and ``d``.