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: Chen Fan <chen.fan.fnst@cn.fujitsu.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>