Document the color.interactive semantics

Document the color.interactive semantics

There are two possible confusions with the color.interactive
description:

  1. the short name "interactive" implies that it covers all
     interactive commands; let's explicitly make it so, even
     though there are no other interactive commands which
     currently use it

  2. Not all parts of "git add --interactive" are controlled
     by color.interactive (specifically, the diffs require
     tweaking color.diff). So let's clarify that it applies
     only to displays and prompts.

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