Doc: Valgrind is not really supported on OS X

According to Valgrind.org, support on OS X 10.8 and 10.9 is
experimental and mostly broken.

Change-Id: I58c4b93d0be45b800881ce55720f92d4b5eb75c8
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: hjk <hjk121@nokiamail.com>