document behavior of empty color name

document behavior of empty color name

Commit 55cccf4bb (color_parse_mem: allow empty color spec,
2017-02-01) clearly defined the behavior of an empty color
config variable. Let's document that, and give a hint about
why it might be useful.

It's important not to say that it makes the item uncolored,
because it doesn't. It just sets no attributes, which means
that any previous attributes continue to take effect.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>