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?

[bcr]

Some grammatical corrections.

share/man/man9/domain.9

251

s/of socket/of the socket/

259

s/is same/is the same/

272

s/notify user/notify the user/

287

s/should allocates and initializes/should allocate and initialize/

293

s/includes/include/

307

d/inform socket/inform the socket/

318

/start to closing/start to close/

323

s/count of socket/count of the socket/

342

s/Finally, assigns the socket address to protocol database./Finally, it assigns the socket address to the protocol database./

345

s/by local/by the local/

347

s/bind socket/bind the socket/

350

s/prepare socket/prepare the socket/

351

s/validates/validate/
s/that incoming/that the incoming/

352

s/does NOT/is NOT/

382

s/be define/be defined/

402

s/by user/by the user/

409

s/terminate/terminates/

[pouria]

@bcr Done.