[gentoo-releng] upgrading istructions for 2004.2

[Gregorio Guidi]

Hi,

Since the release of 2004.2 is impending, I took a little time to put 
gentoo-upgrading.xml in shape. The few lines of instructions that I added to 
deal with 2004.2 are not meant to be final, ideally the releng team should 
substitute with whatever seems appropriate, maybe taken from the release 
notes.

I switched the command to change the profile symlink from referring to 
stackable profiles to non-stackable profiles, since it appears stackable 
profiles are not really used yet.

Thanks
Gregorio