bpf: Update bpf_design_QA.rst to clarify that kprobes is not ABI

This patch updates bpf_design_QA.rst to clarify that the ability to
attach a BPF program to a given point in the kernel code via kprobes
does not make that attachment point be part of the Linux kernel's ABI.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Link: https://lore.kernel.org/r/20220802173913.4170192-1-paulmck@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>