doc: refactor description of color format

doc: refactor description of color format

This is a general cleanup of the description of colors in
git-config, mostly to address inaccuracies and confusion
that had grown over time:

  - you can have many attributes, not just one

  - the discussion flip-flopped between colors and
    attributes; now we discuss everything about colors, then
    everything about attributes

  - many concepts were lumped into the first paragraph,
    making it hard to read, and especially to find the
    actual lists of colors and attributes. I stopped short
    of breaking those out into their own lists, as it seemed
    like an excessive use of vertical screen real estate.

  - we introduced negated attributes, but then the next
    paragraph basically explains how each item starts off
    with no attributes. So why would one need negated
    attributes? We now explain.

  - minor typo, language, and typography fixes

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