Documentation/bisect: revise overall content

Documentation/bisect: revise overall content

Thoroughly revise the "git bisect" manpage, including:

* Beef up the "Description" section.

* Make the first long example less specific to kernel development.

* De-emphasize implementation details in a couple of places.

* Add "(roughly N steps)" in the places where example output is shown.

* Properly markup code within the prose.

* Lots of wordsmithing.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>