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: 68e3c69e (Documentation/glossary: describe "trailer", 2024-11-17) Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name> Signed-off-by: Junio C Hamano <gitster@pobox.com>