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>

Documentation/bpf/bpf_design_QA.rst

@@ -214,6 +214,12 @@ A: NO. Tracepoints are tied to internal implementation details hence they are
 subject to change and can break with newer kernels. BPF programs need to change
 accordingly when this happens.
 
+Q: Are places where kprobes can attach part of the stable ABI?
+--------------------------------------------------------------
+A: NO. The places to which kprobes can attach are internal implementation
+details, which means that they are subject to change and can break with
+newer kernels. BPF programs need to change accordingly when this happens.
+
 Q: How much stack space a BPF program uses?
 -------------------------------------------
 A: Currently all program types are limited to 512 bytes of stack