[3.6] bpo-31458: Clarify that Changelog is built from Misc/NEWS.d directory (GH-3617)

[miss-islington]

(cherry picked from commit 0ec8c4b)

https://bugs.python.org/issue31458

[the-knights-who-say-ni]

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[miss-islington]

@@Mariatta: Backport status check is done, and the result is failure.

[miss-islington]

@@Mariatta: Backport status check is done, and the result is success.

[miss-islington]

@Mariatta: Backport status check is done, and it's a failure ❌ .

[miss-islington]

@Mariatta: Backport status check is done, and it's a success ✅ .

[miss-islington]

Thanks, @Mariatta!