This adds migration instructions for the removed global shared instance
configuration of fcgiwrap.
Adding those explicit messages to the previous options requires moving
the newly defined options from `services.fcgiwrap.*` to
`services.fcgiwrap.instances.*` due to an option namespace clash.
`mkRenamedOptionModule` was not used because the previous options do
not directly map to the new ones. In particular, `user` and `group`
were described as setting the socket's permission, but were actually
setting the process' running user.
Co-authored-by: Minijackson <minijackson@riseup.net>
