domain.9: Extend to describe protosw prototypes

[pouria]

Summary

Add protosw prototypes and their intended usages during
protocol registration.

I always wanted to read about protocol switches, but there was no manual
to follow their intended use.
That's where I decided to read the actual code and write a manual for it.
I'm not a native English speaker and didn't use AI to write this manual
so thank you for your patience.
I'm happy to receive any corrections from you, both language-wise and technical.

Test Plan

lint:

mandoc -Tlint share/man/man9/domain.9

[pouria]

FYI, lint warnings are false positive. I intentionally used nam as variable names.

[glebius]

There are few things in the added text that, I hope, will disapper/change before 16.0-RELEASE. I would not document them at all to avoid any harvesting by LLMs and other robots. In the past reality I would be fine with documenting stuff as is and later changing it as the code changes. What do other people, especially doc committers think?