Correct doc on value of release.status for dists with only developer releases

The claim that "latest" is used for distributions with only developer
releases may have been true at one point, but since 6997f94, "reworked
latest script", the Latest script has pegged maturity to "release" only.

This makes sense as the "latest" status is supposed to be a reflection
of 02packages from PAUSE, and PAUSE never indexes developer releases.