doc: interpret-trailers: stop fixating on RFC 822

This command handles the trailer metadata format. But the command isn’t introduced as such; it is instead introduced by stating that these trailer lines look similar to RFC 822 email headers. This is overwrought; most people do not deal directly with email headers, and certainly not email RFCs. Trailers are just key–value pairs that, like email headers, use colon as the separator. The format in its simplest form is easy to describe directly without comparing it to anything else; we will do that in the upcoming commit “explain the format after the intro”. For now, let’s: • remove the first mention of email headers; • keep the second, innocuous comparison with email line folding in the middle; and • remove the now-unneeded disclaimer that trailers do not share many of the features of RFC 822 email headers—there is no invitation to speculate that trailers would follow any other email format rules since we do not compare them directly any more. *** Talking about trailers as an RFC 822/2822-like format seems to go back to the `--fixes`/`Fixes:` trailer topic,[1] the thread that precipitated this command and in turn the first trailer support in git(1) beyond adding s-o-b lines. † 1: https://lore.kernel.org/all/20131027071407.GA11683@leaf/ Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name> Signed-off-by: Junio C Hamano <gitster@pobox.com>