git-merge documentation: describe how conflict is presented

git-merge documentation: describe how conflict is presented

We took it granted that everybody knows how to read the RCS merge style
conflicts, and did not give illustrations in the documentation.  Now we
are introducing an alternative output style, it is time to document this.

The lack of illustration has been bugging me for a long time.

Signed-off-by: Junio C Hamano <gitster@pobox.com>