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>