doc: interpret-trailers: explain the format after the intro

You need to read the entire “Description” section in order to understand
the full trailer format. But there are many nuances, so that’s fine.
As a starter though we have an introductory example.[1] That turns out
to be crucial; the rest of this section talks about the mechanics of the
command and only incidentally the format itself.

Now, although the example might arguably be self-explanatory, we can
add a little preamble which defines the format in its simplest form as
well as define the most important terms.

Note that we name the “blank line” rule since I want to use that term
every time it comes up. It gets very mildly obfuscated if you call it
a “blank line” in one place[2] and “empty (or whitespace-only) ...” in
another one.[3]

We will define the format of the *key* in the next commit.

† 1: from d57fa7fc (doc: trailer: add more examples in DESCRIPTION,
     2023-06-15)
† 2: `Documentation/git-interpret-trailers.adoc:86` in
     5361983c (The 22nd batch, 2026-03-27)
† 3: `Documentation/git-interpret-trailers.adoc:93` in
     5361983c (The 22nd batch, 2026-03-27)

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>

Documentation/git-interpret-trailers.adoc

@@ -16,7 +16,12 @@ DESCRIPTION
 -----------
 Add or parse trailer metadata at the end of the otherwise
 free-form part of a commit message, or any other kind of text.
-For example, in the following commit message
+
+A _trailer_ in its simplest form is a key-value pair with a colon as a
+separator. A _trailer block_ consists of one or more trailers. The
+trailer block needs to be preceded by a blank line, where a _blank line_
+is either an empty or a whitespace-only line. For example, in the
+following commit message
 
 ------------------------------------------------
 subject