Clarify that the subid delegation can only come from one source.
Moreover, add an example of what might happen if the subid source is NSS
and useradd is executed.
Related: https://github.com/shadow-maint/shadow/issues/331
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:
d5b15f8633