doc: Add link to bpf helpers man page

The bpf-helpers(7) man pages provide an invaluable description of the
functions that an eBPF program can call at runtime. Link them here.

Signed-off-by: Joe Stringer <joe@wand.net.nz>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200813180807.2821735-1-joe@wand.net.nz