doc: shortlog: put back trailer paragraphs

doc: shortlog: put back trailer paragraphs

47beb37b (shortlog: match commit trailers with --group, 2020-09-27)
added the `trailer` bullet point with three paragraphs.[1] Later,
3dc95e09 (shortlog: support arbitrary commit format `--group`s,
2022-10-24) put the single-paragraph bullet point about `format` right
after the first paragraph about `trailer`. That meant that the second
and third paragraphs for `trailer` got moved to `format`.

Move the two paragraphs back to `trailer`. We now also need one blank
line before the final bullet point so that it does not get joined with
the second bullet point.

† 1: Technically the bullet list formatting was immediately fixed to
     include all three paragraphs in 63d24fa0 (shortlog: allow multiple
     groups to be specified, 2020-09-27)

Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>