model: Add a comment on the n_columns field type

model: Add a comment on the n_columns field type

To avoid another bug report like:

  http://bugzilla.openedhand.com/show_bug.cgi?id=2017

with a patch that gets applied without a proper review, resulting in a
bug like:

  http://bugzilla.openedhand.com/show_bug.cgi?id=2032

I should probably add a comment on why on earth we're using an integer
as the n_columns member of the ClutterModelPrivate structure, and why it
is important that it is initialized as -1.