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

This patch updates bpf_design_QA.rst to clarify that the ability to
attach a BPF program to an arbitrary function in the kernel does not
make that function become part of the Linux kernel's ABI.

[ paulmck: Apply Daniel Borkmann feedback. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Link: https://lore.kernel.org/r/20220802173913.4170192-2-paulmck@kernel.orgSigned-off-by: Alexei Starovoitov <ast@kernel.org>

Documentation/bpf/bpf_design_QA.rst

@@ -279,3 +279,15 @@ cc (congestion-control) implementations.  If any of these kernel
 functions has changed, both the in-tree and out-of-tree kernel tcp cc
 implementations have to be changed.  The same goes for the bpf
 programs and they have to be adjusted accordingly.
+
+Q: Attaching to arbitrary kernel functions is an ABI?
+-----------------------------------------------------
+Q: BPF programs can be attached to many kernel functions.  Do these
+kernel functions become part of the ABI?
+
+A: NO.
+
+The kernel function prototypes will change, and BPF programs attaching to
+them will need to change.  The BPF compile-once-run-everywhere (CO-RE)
+should be used in order to make it easier to adapt your BPF programs to
+different versions of the kernel.