man/usermod.8.xml: specify what happens when the current home directory
doesn't exist if using -d and -m options. Moreover, specify what happens
when the group ownership is changed and the uid's don't match in -u and
-g options.
The content of the man pages however is dependent on compile flags. So the
pre-built ones might not fit your version of shadow. To build them yourself use
--enable-man. Furthermore the following build requirements will be needed:
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