Documentation for event filters and formats.

v2 changes: fix a few problems noticed by Randy Dunlap.
Reviewed-by: NLi Zefan <lizf@cn.fujitsu.com>
Acked-by: NRandy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: NTom Zanussi <tzanussi@gmail.com>
LKML-Reference: <1252642431.8016.9.camel@tropicana>
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>