bpf, doc: Update bpf_jit_enable limitation for CONFIG_BPF_JIT_ALWAYS_ON

When CONFIG_BPF_JIT_ALWAYS_ON is enabled, kernel has limitation for
bpf_jit_enable, so it has fixed value 1 and we cannot set it to 2
for JIT opcode dumping; this patch is to update the doc for it.

Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>