tracing/documentation: Document dynamic ftracer internals

Add more details to the dynamic function tracing design implementation.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
LKML-Reference: <1279610015-10250-1-git-send-email-vapier@gentoo.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>