config: document --type=color output is a complete line

config: document --type=color output is a complete line

Even though the newer "--type=color" option to "git config" is meant
to be upward compatible with the traditional "--get-color" option,
unlike the latter, its output is not an incomplete line that lack
the LF at the end.  That makes it consistent with output of other
types like "git config --type=bool".

Document it, as it sometimes surprises unsuspecting users.

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