rev-list documentation: clarify the two parts of history simplification One set of options and parameters determine what commits are involved in the simplification process, and another set of options determine how the simplification is done. Clarify their distinction at the beginning. Signed-off-by: Santi Béjar <santi@agolina.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>