bpf, docs: Use IETF format for field definitions in instruction-set.rst

bpf, docs: Use IETF format for field definitions in instruction-set.rst

In preparation for publication as an IETF RFC, the WG chairs asked me
to convert the document to use IETF packet format for field layout, so
this patch attempts to make it consistent with other IETF documents.

Some fields that are not byte aligned were previously inconsistent
in how values were defined.  Some were defined as the value of the
byte containing the field (like 0x20 for a field holding the high
four bits of the byte), and others were defined as the value of the
field itself (like 0x2).  This PR makes them be consistent in using
just the values of the field itself, which is IETF convention.

As a result, some of the defines that used BPF_* would no longer
match the value in the spec, and so this patch also drops the BPF_*
prefix to avoid confusion with the defines that are the full-byte
equivalent values.  For consistency, BPF_* is then dropped from
other fields too.  BPF_<foo> is thus the Linux implementation-specific
define for <foo> as it appears in the BPF ISA specification.

The syntax BPF_ADD | BPF_X | BPF_ALU only worked for full-byte
values so the convention {ADD, X, ALU} is proposed for referring
to field values instead.

Also replace the redundant "LSB bits" with "least significant bits".

A preview of what the resulting Internet Draft would look like can
be seen at:
https://htmlpreview.github.io/?https://raw.githubusercontent.com/dthaler/ebp
f-docs-1/format/draft-ietf-bpf-isa.html

v1->v2: Fix sphinx issue as recommended by David Vernet

Signed-off-by: Dave Thaler <dthaler1968@gmail.com>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20240301222337.15931-1-dthaler1968@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>