docs/bpf: Document some special sdiv/smod operations

Patch [1] fixed possible kernel crash due to specific sdiv/smod operations
in bpf program. The following are related operations and the expected results
of those operations:
  - LLONG_MIN/-1 = LLONG_MIN
  - INT_MIN/-1 = INT_MIN
  - LLONG_MIN%-1 = 0
  - INT_MIN%-1 = 0

Those operations are replaced with codes which won't cause
kernel crash. This patch documents what operations may cause exception and
what replacement operations are.

  [1] https://lore.kernel.org/all/20240913150326.1187788-1-yonghong.song@linux.dev/



Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20241107170924.2944681-1-yonghong.song@linux.dev


Signed-off-by: Alexei Starovoitov <ast@kernel.org>