ftrace: ftrace.txt updates

This patch includes ftrace.txt updates that address (mostly) comments from
Andrew Morton. It also includes updates that were suggested by Randy
Dunlap, John Kacur and David Teigland.
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>