ftrace: update txt document

Impact: Documentation update only

A lot of changes have gone into ftrace. This patch updates
the ftrace.txt document.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>