Doc: show example scissors line

Doc: show example scissors line

The text tries to say the code accepts many variations that look remotely
like scissors and perforation marks, but gives too little detail for users
to decide what is and what is not taken as a scissors line for themselves.
Instead of describing the heuristics more, just spell out what will always
be accepted, namely "-- >8 --", as it would not help users to give them
more choices and flexibility and be "creative" in their scissors line.

Signed-off-by: Evan Gates <evan.gates@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>