Documentation/trace: Correcting and extending tracepoint documentation

The sample missed the moving of the header files into the events subdirectory.
I've also extended it based on the existing headers, and mentioned the tiny
but important role of CREATE_TRACE_POINTS.

Signed-off-by: Zoltan Kiss <zoltan.kiss@citrix.com>
Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>