bpf: Update bpf_design_QA.rst to clarify that BTF_ID does not ABIify a function

bpf: Update bpf_design_QA.rst to clarify that BTF_ID does not ABIify a function

This patch updates bpf_design_QA.rst to clarify that mentioning a function
to the BTF_ID macro does not make that function become part of the Linux
kernel's ABI.

Suggested-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Link: https://lore.kernel.org/r/20220802173913.4170192-3-paulmck@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>