Show a opt-in warning for undocumented open / public code blocks

[CodebyCR]

New Issue Checklist

• I've Updated SwiftLint to the latest version.
• I've searched for existing GitHub issues.

Feature or Enhancement Proposal

Note

In the text below, code blocks means classes, structs, actors, protocols, enums, and methods.

Show a opt-in warning for undocumented open / public code blocks

This could increase the frequency of documented code that you want to share with other developers.

For example, imagine you've written a Swift package that you want to make publicly available. You want to document all the methods
that users of your package can use. These are all open and public code blocks.

Now imagine your package has more than 150 files.
It could easily happen that you forget to document some of your public or open code blocks.
But if you use an opt-in warning for this, you can step through your warnings to fix them.

[SimplyDanny]

What about the missing_docs rule?