bpo-34956: edit and format better NEWS item in 3.9.0b1 changelog (GH-20255)

Author: ned-deily

Misc/NEWS.d/3.9.0b1.rst

@@ -800,12 +800,13 @@ it's not coming from Windows API.
 .. nonce: 35IcGF
 .. section: macOS
 
-_tkinter now builds and links with non-system Tcl and Tk frameworks if they
-are installed in /Library/Frameworks as had been the case on older releases
-of macOS. If a macOS SDK is explicitly configured, by using ./configure
---enable-universalsdk= or -isysroot, only a Library/Frameworks directory in
-the SDK itself is searched. The default behavior can still be overridden
-with configure --with-tcltk-includes and --with-tcltk-libs.
+When building Python on macOS from source, ``_tkinter`` now links with
+non-system Tcl and Tk frameworks if they are installed in
+``/Library/Frameworks``, as had been the case on older releases
+of macOS. If a macOS SDK is explicitly configured, by using
+``--enable-universalsdk=`` or ``-isysroot``, only the SDK itself is
+searched. The default behavior can still be overridden with
+``--with-tcltk-includes`` and ``--with-tcltk-libs``.
 
 ..