tiny bit more clarity about the format specifiers

[ChrisBarker-NOAA]

See this thread on python-dev:

https://mail.python.org/pipermail/python-dev/2017-December/151282.html

NOTE: PEPs aren't really supposed to be documentation for end users, so maybe there's not point in this little clarification. And some clarification has been added to the main Python docs.

But why not? People do come here to learn about new features.

[the-knights-who-say-ni]

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[ChrisBarker-NOAA]

I'll re-submit from a different gitHub ID that I signed the CLA under.