Document the 'no_default_update' keyword of theano.function in the online doc.

doc/library/compile/function.txt

@@ -104,7 +104,6 @@ Reference
 
     :param updates: expressions for new :class:`SharedVariable` values
 
-
     :type givens: iterable over pairs (Var1, Var2) of Variables.
        List, tuple or dict.  The Var1
        and Var2 in each pair must have the same Type.
@@ -112,6 +111,13 @@ Reference
     :param givens: specific substitutions to make in the
       computation graph (Var2 replaces Var1).
 
+    :type no_default_updates: either bool or list of Variables
+    :param no_default_updates:
+        if True, do not perform any automatic update on Variables.
+        If False (default), perform them all.
+        Else, perform automatic updates on all Variables that are
+        neither in ``updates`` nor in ``no_default_updates``.
+
     :param name: an optional name for this function.
       The profile mode will print the time spent in this function.
 
@@ -138,6 +144,13 @@ Reference
     if you give two update expressions for the same SharedVariable input (that
     doesn't make sense).
 
+    If a SharedVariable is not given an update expression, but has a
+    ``default_update`` member containing an expression, this expression
+    will be used as the update expression for this variable.  Passing
+    ``no_default_updates=True`` to ``function`` disables this behavior
+    entirely, passing ``no_default_updates=[sharedvar1, sharedvar2]``
+    disables it for the mentionned variables.
+
     Regarding givens: Be careful to make sure that these substitutions are
     independent--behaviour when Var1 of one pair appears in the graph leading
     to Var2 in another expression is undefined.  Replacements specified with