add example to ModuleInfo documentation

[adamdruppe]

Follow-up to the 100% correct information written by Walter, reviewed by me, and merged by Razvan in #3464 to demonstrate to the reader how things are supposed to work.

[dlang-bot]

Thanks for your pull request and interest in making D better, @adamdruppe! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

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[adamdruppe]

Force pushed with more information, to show how these modules must work with any combination of -betterC thanks to the ModuleInfo generation rules. I think this is complete now, ping @RazvanN7 @maxhaton

[WalterBright]

The prose needs to explain what is happening, and the tone of the existing prose should be done in the usual dry tone of specifications.