doc: interpret-trailers: don’t use deprecated config

doc: interpret-trailers: don’t use deprecated config

`command` has been deprecated since commit c364b7ef51 (trailer: add new
.cmd config option, 2021-05-03).

Use the commit message of c364b7ef51 as a guide to replace the use of
`$ARG` and to use a script instead of an inline command.[1] Also,
explicitly trigger the command by passing in `--trailer=see`, since
this config is not automatically used.[2]

[1]: “Instead of "$ARG", users can refer to the value as positional
   argument, $1, in their scripts.”
[2]: “At the same time, in order to allow `git interpret-trailers` to
   better simulate the behavior of `git command -s`,
   'trailer.<token>.cmd' will not automatically execute.”

Acked-by: ZheNing Hu <adlternative@gmail.com>
Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>