tracing: add comments to explain TRACE_EVENT out of protection

The commit:
  commit 5ac35daa93
  Author: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
  tracing/events: fix the include file dependencies

Moved the TRACE_EVENT out of the ifdef protection of tracepoints.h
but uses the define of TRACE_EVENT itself as protection. This patch
adds comments to explain why.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

include/linux/tracepoint.h

@@ -180,7 +180,15 @@ static inline void tracepoint_synchronize_unregister(void)
 }
 
 #define PARAMS(args...) args
-#endif
+
+#endif /* _LINUX_TRACEPOINT_H */
+
+/*
+ * Note: we keep the TRACE_EVENT outside the include file ifdef protection.
+ *  This is due to the way trace events work. If a file includes two
+ *  trace event headers under one "CREATE_TRACE_POINTS" the first include
+ *  will override the TRACE_EVENT and break the second include.
+ */
 
 #ifndef TRACE_EVENT
 /*
@@ -287,4 +295,5 @@ static inline void tracepoint_synchronize_unregister(void)
 
 #define TRACE_EVENT(name, proto, args, struct, assign, print)	\
 	DECLARE_TRACE(name, PARAMS(proto), PARAMS(args))
-#endif
+
+#endif /* ifdef TRACE_EVENT (see note above) */