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

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.

† 1: in commit “explain the format after the intro”

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>