doc: interpret-trailers: commit to “trailer block” term

We chose to introduce the term “trailer block” into the documentation a few commits ago.[1] It is used in the code though, so it is not a newly invented term. That term was useful to explain where the trailers are found (they *trail* the message). But it is also useful here, where we explain how trailers are added to existing messages, how trailer blocks are found (beyond the simple case in the introduction), and how the end of the message is found. Also note that we simplify the “blank line” point. The text says: A blank line will be added before the new trailer if there isn't one already. But this isn’t quite coherent. The previous sentence says “If there is no existing trailer”, so we are in one of these modes: 1. discussing trailer blocks in general; or 2. discussing creating a new trailer block in particular. If (1), then we shouldn’t add a blank line before the new trailer if there exists a trailer block already. And if (2), then the “if there isn’t one already” is redundant.[2] So just talking about the higher- level “trailer block” simplifies the text, since we don’t have to worry about the different contexts that *trailers* can find themselves in. † 1: in commit “explain the format after the intro” † 2: Note that non-trailer lines don’t matter here; if you have a trailer block consisting of `(cherry picked from commit <commit>)`, then you still shouldn’t insert a blank line before the new trailer since that would create a new trailer block Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name> Signed-off-by: Junio C Hamano <gitster@pobox.com>