Improvements to `JULIA_CPU_TARGET` and its documentation

[giordano]

Follow up from #54471

Great to have this improved but I agree with Mose that this is not clear:

basically pkgimages can only be more specific than the base system image

Is an error returned if the user tries to make a package image for a totally different target than the base image? I don't fully understand this code but it seems that there is no check for that case. Would it be feasible to implement such a check? Otherwise this feels like a footgun. I could also update the docs here and here but I don't think that would be enough.

Originally posted by @Octogonapus in #54471 (comment)