bpf, docs: specify which BPF_ABS and BPF_IND fields were zero

Specifying which fields were unused allows IANA to only list as deprecated
instructions that were actually used, leaving the rest as unassigned and
possibly available for future use for something else.

Signed-off-by: Dave Thaler <dthaler1968@gmail.com>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20240221175419.16843-1-dthaler1968@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: Dave Thaler

Documentation/bpf/standardization/instruction-set.rst

@@ -658,6 +658,7 @@ Legacy BPF Packet access instructions
 BPF previously introduced special instructions for access to packet data that were
 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.
+mode modifier of BPF_ABS or BPF_IND.  The 'dst_reg' and 'offset' fields were
+set to zero, and 'src_reg' was set to zero for BPF_ABS.  However, these
+instructions are deprecated and should no longer be used.  All legacy packet
+access instructions belong to the "legacy" conformance group.