doc: interpret-trailers: stop fixating on RFC 822

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>