nspawn: improve man page (#3577)

nspawn: improve man page (#3577)

This change documents the existance of the systemd-nspawn@.service template
unit file, which was previously not mentioned at all. Since the unit file uses
slightly different default than nspawn invoked from the command line, these
defaults are now explicitly documented too.

A couple of further additions and changes are made, too.

Replaces: #3497