Documentation/trace/tracepoints.txt: 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: Stefan Raspl <raspl@linux.vnet.ibm.com>
Cc: Rob Landley <rob@landley.net>
Cc: Jiri Kosina <jkosina@suse.cz>
Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Zoltan Kiss <zoltan.kiss@citrix.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/trace/tracepoints.txt

@@ -114,3 +114,8 @@ core kernel image or in modules.
 If the tracepoint has to be used in kernel modules, an
 EXPORT_TRACEPOINT_SYMBOL_GPL() or EXPORT_TRACEPOINT_SYMBOL() can be
 used to export the defined tracepoints.
+
+Note: The convenience macro TRACE_EVENT provides an alternative way to
+      define tracepoints. Check http://lwn.net/Articles/379903,
+      http://lwn.net/Articles/381064 and http://lwn.net/Articles/383362
+      for a series of articles with more details.