tracing: Update the stage 3 of trace event macro comment

Update the comment of the 3rd stage of trace event macro
expansion code. Now there are 2 macros makes different
trace_raw_output_<call>() functions.

Link: https://lkml.kernel.org/r/160277371605.29307.8586817119278606720.stgit@devnote2



Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>