documentation: Add links to TRACE_EVENT documentation

Existing tracepoint documentation doesn't mention the popular
TRACE_EVENT
macro. Since an excellent series of articles on proper usage already
exists, respective links are added to the existing documentation.
Signed-off-by: NStefan Raspl <raspl@linux.vnet.ibm.com>
Acked-by: NRob Landley <rob@landley.net>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>