tracing: Add example and documentation for new __vstring() macro

Update the sample trace events to include an example that uses the new
__vstring() helpers for TRACE_EVENTS.

Link: https://lkml.kernel.org/r/20220715175555.16375a3b@gandalf.local.home



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