[PATCH 06/18 v3] tracing: Update trace-event-sample with TRACE_SYSTEM_VAR documentation

From: "Steven Rostedt (Red Hat)" 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

[PATCH 06/18 v3] tracing: Update trace-event-sample with TRACE_SYSTEM_VAR documentation

From: Steven Rostedt (Red Hat) rost...@goodmis.org 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