doc: improvements to domain.markdown copy

[jasnell]

General improvements to the domain.markdown copy including
improved examples.

[jasnell]

@nodejs/documentation @misterdjules

[Qard]

Singular module?

[Qard]

Some minor nits, but LGTM when those are addressed.

[jasnell]

@Qard ... fixed!

[misterdjules]

It might help to mention why the master process exits with an error in this case. Unless readers are familiar with the cluster module, it's not crystal clear.

[misterdjules]

@jasnell Thank you for doing that!

I'm not a fan of having the first/only examples tied to using the cluster module. Not because the two don't go well together, but because it adds to the cognitive load when reading the documentation: one has to be familiar with the cluster module to understand the documentation about domains. I understand though that changing that is not in the scope of this PR. So please feel free to ignore my comments on this topic for now :)

Once the rest of my comments are addressed I'll take another look, but this already looks like a great improvement.

[jasnell]

@misterdjules .. thanks for the comments! I'll update shortly