Some Structure for MutableType doc paragraph

Not sure if I'm using the correct heading markup:

diff -r e15fa0342d2ac83414c563abd8fd478251d4d35f lib/sqlalchemy/types.py
--- a/lib/sqlalchemy/types.py   Thu Nov 18 20:12:24 2010 -0500
+++ b/lib/sqlalchemy/types.py   Sat Nov 20 11:52:40 2010 +0000
@@ -544,6 +544,9 @@
     and compare function for values of this type - implementing
     subclasses should override these appropriately.

+    Performance Impact
+    ^^^^^^^^^^^^^^^^^^
+
     The usage of mutable types has significant performance
     implications when using the ORM. In order to detect changes, the
     ORM must create a copy of the value when it is first
@@ -564,6 +567,9 @@
     negligible.  In most cases it's likely that the convenience allowed 
     by "mutable" change detection outweighs the performance penalty.

+    Alternative Usage (mutable=False)
+    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
     It is perfectly fine to represent "mutable" data types with the
     "mutable" flag set to False, which eliminates any performance
     issues. It means that the ORM will only reliably detect changes

Would also be good if under the Alternative section, an example could be added of how to set the flag, e.g.

    PickleType(mutable=False)

EoghanM

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