bpf, libbpf: clarify bump in libbpf version info

The current documentation suggests that we would need to bump the
libbpf version on every change. Lets clarify this a bit more and
reflect what we do today in practice, that is, bumping it once per
development cycle.

Fixes: 76d1b894 ("libbpf: Document API and ABI conventions")
Reported-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>