doc: interpret-trailers: join new-trailers again

There are three paragraphs that talk about how a new trailer is added. But the first one is separated from the other two by two paragraphs about how `key-alias` can make using `--trailer` more convenient. This short how-to does not follow thematically from the previous paragraph, and can wait until we have fully described how a new trailer is added. So let’s move the three paragraphs about the new-trailer topic together and move the how-to paragraphs after that. *** Let’s now review the history of the document. Even if the document is not quite correct in its current state, just doing the apparently obvious edit without considering the history does not respect the effort that went into changing the document in the past. These three paragraphs were originally next to each other, in the first version of the doc.[1] But extra sentences about this how-to topic was added to the first paragraph nine years later:[2] [...] `': '` (one colon followed by one space). For convenience, the <token> can be a shortened string key (e.g., "sign") instead of the full string which should [...] And then it was split into it’s own paragraph a little later.[3] This evolution shows, in my opinion, that this how-to never followed thematically from the existing topic. Which means that there is nothing that was potentially lost to time that we need to restore or respect. † 1: dfd66ddf (Documentation: add documentation for 'git interpret-trailers', 2014-10-13) † 2: eda2c44c (doc: trailer: mention 'key' in DESCRIPTION, 2023-06-15) † 3: 6ccbc667 (trailer doc: <token> is a <key> or <keyAlias>, not both, 2023-09-07) Suggested-by: D. Ben Knoble <ben.knoble+github@gmail.com> Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name> Signed-off-by: Junio C Hamano <gitster@pobox.com>