cmd/stress: flush out Usage doc

Provide more context when you run `stress -h` besides just the flags.

Change-Id: I9dbe7ba2b7178dd7a542d8c4c29bf79999a38234
Reviewed-on: https://go-review.googlesource.com/44810
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
1 file changed