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>

Documentation/networking/filter.txt

@@ -483,6 +483,12 @@ Example output from dmesg:
 [ 3389.935851] JIT code: 00000030: 00 e8 28 94 ff e0 83 f8 01 75 07 b8 ff ff 00 00
 [ 3389.935852] JIT code: 00000040: eb 02 31 c0 c9 c3
 
+When CONFIG_BPF_JIT_ALWAYS_ON is enabled, bpf_jit_enable is permanently set to 1 and
+setting any other value than that will return in failure. This is even the case for
+setting bpf_jit_enable to 2, since dumping the final JIT image into the kernel log
+is discouraged and introspection through bpftool (under tools/bpf/bpftool/) is the
+generally recommended approach instead.
+
 In the kernel source tree under tools/bpf/, there's bpf_jit_disasm for
 generating disassembly out of the kernel log's hexdump: