doc: interpret-trailers: replace “lines” with “metadata”

We removed the initial comparison to email headers in the previous
commit. Now the introduction paragraph just says “trailer lines”, and
the only hint that this is metadata/structured information is the
“otherwise free-form” phrase.

Let’s replace “lines” with “metadata” since that is their purpose.
This also makes the introduction more consistent with how I chose
to define trailers in the glossary:[1] “Key-value metadata”. (We will
introduce “key–value” in the upcoming commit “explain the format after
the intro”.)

† 1: 68e3c69 (Documentation/glossary: describe "trailer", 2024-11-17)

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

Author: LemmingAvalanche

Documentation/git-interpret-trailers.adoc

@@ -14,7 +14,7 @@ git interpret-trailers [--in-place] [--trim-empty]
 
 DESCRIPTION
 -----------
-Add or parse _trailer_ lines at the end of the otherwise
+Add or parse trailers metadata at the end of the otherwise
 free-form part of a commit message. For example, in the following commit
 message