Fixing useradd feature documentation

The manages password and manages password age features of useradd were
inaccurately documented.

Support to manage passwords was added for 'useradd' in 0.22.4
Support to manage password ages was added for 'useradd'  in 2.6.2

This change retrofits all documents to reflect the appropriate features at the
time they were introduced
Merge pull request #142 from jjulien/useradd_features

Fixing useradd feature documentation