Add missing documentation about group item

[asb-capfan]

The book "Mastering Perl/Tk" briefly introduces the canvas group item in chapter 9 (page 212ff). There also is an example here: http://www.perlmonks.org/?node_id=602272

The documentation about the group item is missing in the POD of Tk::Canvas.

Could we please add it? Is it even a supported feature? I didn't find any information about it here: http://www.tcl.tk/man/tcl/TkCmd/canvas.htm