documentation: user-manual: update "using-bisect" section

documentation: user-manual: update "using-bisect" section

Since version 1.5.6 "git bisect" doesn't use a "bisect" branch any
more, but the user manual had not been updated to reflect this.

So this patch does that and while at it also adds a few words about
"git bisect skip" and points user to the "git bisect" man page for
more information.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>