t/lib-rebase: improve documentation of set_fake_editor()

t/lib-rebase: improve documentation of set_fake_editor()

Firstly, make it reflect better what actually happens. Not omitting some
possibilities makes it easier to fully exploit them, and not
contradicting the implementation makes it easier to grok and thus modify
the code.

Secondly, improve the overall structure, putting more general info
further up.

Thirdly, document `merge`, `fakesha`, and `break`, which were previously
omitted entirely.

Signed-off-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>