bpo-24896: Added effect of re.ASCII and reworded slightly

[bricsuc]

This is the Python 3 counterpart to the case-insensitive flag doc addition.

[the-knights-who-say-ni]

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[mention-bot]

@bricsuc, thanks for your PR! By analyzing the history of the files in this pull request, we identified @birkenfeld, @tiran and @serhiy-storchaka to be potential reviewers.

[bricsuc]

@gpshead

[gpshead]

@bricsuc has signed the CLA and was sitting next to me while making these changes.

[serhiy-storchaka]

As I mentioned on the bugtracker, the new wording still is not correct.