[2.7] bpo-24896: added re.UNICODE effects to insensitive flags.

[bricsuc]

bpo-24896: re.UNICODE adds useful functionality for re.IGNORECASE; added blurbs to both flag descriptions to point this out.

[the-knights-who-say-ni]

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[bricsuc]

@bricsuc

[gpshead]

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