Add documentation about TRACE_SYSTEM needing to be alpha-numeric or with
underscores, and that if it is not, then the use of TRACE_SYSTEM_VAR is
required to make something that is.

An example of this is shown in samples/trace_events/trace-events-sample.h

Link: http://lkml.kernel.org/r/20150403013802.220157513@goodmis.orgReviewed-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Tested-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>