bpo-41100: clarify NEWS item about macOS 11 support (GH-21174)

(cherry picked from commit 2f168c6)

Co-authored-by: Ned Deily <nad@python.org>

Misc/NEWS.d/next/macOS/2020-06-24-13-51-57.bpo-41100.mcHdc5.rst

@@ -1 +1,7 @@
-Support macOS 11 when building.
+Fix configure error when building on macOS 11.
+Note that the current Python release was released
+shortly after the first developer preview of macOS
+11 (Big Sur); there are other known issues with
+building and running on the developer preview.
+Big Sur is expected to be fully supported in a
+future bugfix release of Python 3.8.x and with 3.9.0.