[pyflakes] Add fix safety section to docs (F601, F602)

[Kalmaegi]

Summary

add fix safety section to repeated_keys_docs, for #15584

[ntBre]

Thanks! I also tested this one, and it definitely removes comments.

It could also technically be unsafe if the values have side effects:

foo = {
    2: print(2),
    2: print(2),
}

The rule only offers a fix if the values are equal, but it doesn't consider potential side effects.

I think mentioning the comments is sufficient, though. This example is pretty contrived, and the comment case seems most common.

[github-actions]

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[ntBre]

On second thought, I went ahead and added the mention of side effects, just in case. I'll merge once CI finishes!