bpf, docs: Clarify which legacy packet instructions existed

As discussed on the BPF IETF mailing list (see link), this patch updates
the "Legacy BPF Packet access instructions" section to clarify which
instructions are deprecated (vs which were never defined and so are not
deprecated).
Signed-off-by: Dave Thaler <dthaler1968@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: David Vernet <void@manifault.com>
Link: https://mailarchive.ietf.org/arch/msg/bpf/5LnnKm093cGpOmDI9TnLQLBXyys
Link: https://lore.kernel.org/bpf/20240131033759.3634-1-dthaler1968@gmail.com

Documentation/bpf/standardization/instruction-set.rst

@@ -630,7 +630,9 @@ Legacy BPF Packet access instructions
 -------------------------------------
 
 BPF previously introduced special instructions for access to packet data that were
-carried over from classic BPF. However, these instructions are
+carried over from classic BPF. These instructions used an instruction
+class of BPF_LD, a size modifier of BPF_W, BPF_H, or BPF_B, and a
+mode modifier of BPF_ABS or BPF_IND.  However, these instructions are
 deprecated and should no longer be used.  All legacy packet access
 instructions belong to the "legacy" conformance group instead of the "basic"
 conformance group.