Document "standard" conventions for error messages

These are currently documented in the API guidelines:

https://rust-lang.github.io/api-guidelines/interoperability.html#error-types-are-meaningful-and-well-behaved-c-good-err

I think it makes sense to uplift this guideline (in a milder form) into
std docs. Printing and producing errors is something that even
non-expert users do frequently, so it is useful to give at least some
indication of what a typical error message looks like.

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