From one piece Why Rust is different, with Alice Ryhl 2 beliefs, in the piece's order there
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Their words
And what this is is that in the guide level explanation, you explain your feature as if you were writing a guide as if the feature already existed. And in the reference level explanation, you explain your feature again as if it already existed, but as if it would be in the language reference instead of a tutorial.
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Their words
Now the thing is you can of course write examples in your documentation and Rust makes all examples into tests. This means that if you change the underlying code now your test fails. And so this means that you can't even you can't forget to update your examples in your documentation.