Write some docs for functions in pretty.h.
Take it as a first draft, they would be changed later.
Signed-off-by: Olga Telezhnaia <olyatelezhnaya@gmail.com>
Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>