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

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>