docs: update status --porcelain format

docs: update status --porcelain format

The --porcelain format was originally identical to the
--short format, but designed to be stable as the short
format changed. Since this was written, the short format
picked up a few incompatible niceties, but this description
was never changed.

Let's mention the differences. While we're at it, let's add
some sub-section headings to make the "output" section a
little easier to navigate.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>