ftrace/docs: Add documentation on how to use ftrace from within the kernel

With the coming removal of jprobes, using ftrace callbacks is one of the
utilities that replace the jprobes functionality. Having a document that
explains how to use ftrace as such will help in the transition from jprobes
to ftrace. This document is for kernel developers that require attaching a
callback to a function within the kernel.

Link: http://lkml.kernel.org/r/150724519527.5014.10207042218696587159.stgit@devboxSigned-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
[jc: fixed one formatting issue that broke the docs build]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>