bpf: update perf event helper functions documentation

This commit updates documentation of the bpf_perf_event_output and
bpf_perf_event_read helpers to match their implementation.

Signed-off-by: Teng Qin <qinteng@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>