Emphasize the file naming convention and why, add a section about "instancing"

[netchild]

Summary

The reason for emphasizing the file naming convention is at

https://lists.freebsd.org/archives/freebsd-ports/2024-July/006342.html

Short:

• the filename shall be the same as the content of the name variable inside the script
• PROVIDE shall be the same as the content of the name variable inside the script
• this fixes the use of the scripts with service jails

While I'm here, add a section about how to create a rc script which does instancing of services.

[pauamma_gundo.com]

English LGTM and the changes look sensible to not-an-expert me.