tracing: Add documentation for trace commands mod, traceon/traceoff

The mod command went in as commit
64e7c440

The traceon/traceoff commands went in as commit
23b4ff3a

Signed-off-by: Chase Douglas <chase.douglas@canonical.com>
LKML-Reference: <1272045759-32018-1-git-send-email-chase.douglas@canonical.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>