man: document socket requirement for systemd-socket-proxyd (#6535)

man: document socket requirement for systemd-socket-proxyd (#6535)

Without this requirement, if proxy-to-nginx.socket was down, and the sysadmin
were to do:

    systemctl start proxy-to-nginx.service

then the service would come up without a configured socket, which doesn't make
sense.  Normally this isn't how we expect a socket-activated service to start,
but it's possible for an admin to do this (if the .socket were already running,
the systemd-socket-proxyd process will start effectively idle).  But the
.service shouldn't end up in a broken state if the .socket isn't already
listening.

Adding the explicit Requires: should ensure that an admin with this
configuration state can't accidentally break their system.