GtkTreeRowReference requested for Tree and List widget Overview

[Kang Jeong-Hee <Keizi mail co kr>]

Hi.

the tree and list widget overview describe
brief of tree widget and related objects.
and there's a missing.

on the other hand, text widget overview
describe GtkTextMark:
it does equivalent functionality to GtkTreeRowRefernece, I think.

but I couldn't find the useful object before scanning all API reference
on gtk.org/api.

so I got a ask for you to add GtkTreeRowReference on tree overview.

thanks.