bpf: doc: update answer for 32-bit subregister question

There has been quite a few progress around the two steps mentioned in the
answer to the following question:

  Q: BPF 32-bit subregister requirements

This patch updates the answer to reflect what has been done.

v2:
 - Add missing full stop. (Song Liu)
 - Minor tweak on one sentence. (Song Liu)

v1:
 - Integrated rephrase from Quentin and Jakub

Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>