docs: Change suffix for desktop applications to support non-transient services

docs: Change suffix for desktop applications to support non-transient services

One problem found with the current draft specification is we can't have
an application provide a non-transient systemd service file in a way
that is spec compliant as the service name currently needs to end in a
random token defined by the launcher.

This came up when trying to put DBus activated services into the correct
cgroup. There isn't enough metadata in the DBus service file to know the
correct application ID, and the most intuitive fix is for those
applications to just specify the SystemdService file in the existing
system. They're generally unique for a given user session anyway so
don't need a separate cgroup identifier.

This changes the spec for RANDOM to be optional for services.

It also changes the separator between in services to act like templates.
Ultimately that's what we're trying to recreate with the RANDOM token of
the systemd service and it's a better fit. It's needed as otherwise with
launcher and the random ident being both optional it would be impossible
to get the application ID reliably.

Scopes are unchanged as they don't support templates.