trace: docs: add trace file description

When user used the trace print command from docs/tracing.txt:
  ./scripts/simpletrace.py trace-events trace-*

the user maybe be misled by the "trace-*", because if user
directly copy the comand line to run, there alway print the
bored message:
"usage: ./scripts/simpletrace.py <trace-events> <trace-file>"

then we should describe that the "trace-*" represented.
Signed-off-by: NChen Fan <chen.fan.fnst@cn.fujitsu.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

docs/tracing.txt

@@ -23,7 +23,7 @@ for debugging, profiling, and observing execution.
 
 4. Pretty-print the binary trace file:
 
-    ./scripts/simpletrace.py trace-events trace-*
+    ./scripts/simpletrace.py trace-events trace-* # Override * with QEMU <pid>
 
 == Trace events ==