perf script: Add 'bpf-output' field to usage message

This adds the 'bpf-output' field to the perf script usage message, and docs.

Signed-off-by: Brendan Gregg <bgregg@netflix.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1470192469-11910-4-git-send-email-bgregg@netflix.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>