bpf: clarify description for CONFIG_BPF_EVENTS

PERF_EVENT_IOC_SET_BPF supports uprobes since v4.3, and tracepoints
since v4.7 via commit 04a22fae ("tracing, perf: Implement BPF
programs attached to uprobes"), and commit 98b5c2c6

 ("perf, bpf:
allow bpf programs attach to tracepoints") respectively.

Signed-off-by: Peter Wu <peter@lekensteyn.nl>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>