Rephrase and expand empty_enum documentation.

[kpreid]

• Remove incorrect claim that “wrappers around it are the conventional way to define an uninhabited type”.
• Discuss why one would use !, a newtype struct, or keep the enum.
• Add links to relevant documentation.

Before writing this change, I asked the community via IRLO and Zulip for feedback. The broad consensus seemed to me to be that in a world where both never_type and min_exhaustive_patterns are stable and therefore available for general use, we might want to ! or newtypes of it — but it's certainly not “conventional” yet. Therefore, I've removed “conventional” and added a discussion of the pros and cons of different choices.

changelog: [empty_enum]: expanded documentation

[rustbot]

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[Manishearth]

@bors r+

thanks!

[bors]

📌 Commit cfa150b has been approved by Manishearth

It is now in the queue for this repository.

[bors]

⌛ Testing commit cfa150b with merge 76856ff...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Manishearth
Pushing 76856ff to master...