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: 76d1b89 ("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>

tools/lib/bpf/README.rst

@@ -111,6 +111,7 @@ starting from ``0.0.1``.
 
 Every time ABI is being changed, e.g. because a new symbol is added or
 semantic of existing symbol is changed, ABI version should be bumped.
+This bump in ABI version is at most once per kernel development cycle.
 
 For example, if current state of ``libbpf.map`` is: