doc: explain the use of color.pager

doc: explain the use of color.pager

The current documentation for color.pager is technically correct, but
slightly misleading and doesn't really clarify the purpose of the
variable. As explained in the original thread which added it:

  https://lore.kernel.org/git/E1G6zPH-00062L-Je@moooo.ath.cx/

the point is to deal with pagers that don't understand colors. And hence it
being set to "true" is necessary for colorizing output to the pager, but
not sufficient by itself (you must also have enabled one of the other
color options, though note that these are set to "auto" by default these
days).

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