File out news blurb explaining effect for users.

Misc/NEWS.d/next/IDLE/2017-08-24-13-48-16.bpo-27099.rENefC.rst

@@ -1,2 +1,20 @@
-Convert IDLE's built-in 'extensions' to regular features. Inital patch by
-Charles Wohlganger.
+Convert IDLE's built-in 'extensions' to regular features.
+
+About 10 IDLE features were implemented as supposedly optional
+extensions.  Their different behavior could be confusing or worse for
+users and not good for maintenance.  Hence the conversion.
+
+The main difference for users is that user configurable key bindings
+for builtin features are now handled uniformly.  Now, editing a binding
+in a keyset only affects its value in the keyset.  All bindings are
+defined together in the system-specific default keysets in config-
+extensions.def.  All custom keysets are saved as a whole in config-
+extension.cfg.  All take effect as soon as one clicks Apply or Ok.
+
+The affected events are '<<force-open-completions>>', '<<expand-word>>',
+'<<force-open-calltip>>', '<<flash-paren>>', '<<format-paragraph>>',
+'<<run-module>>', '<<check-module>>', and '<<zoom-height>>'.  Any
+(global) customizations made before 3.6.3 will not affect their keyset-
+specific customization after 3.6.3. and vice versa.
+
+Inital patch by Charles Wohlganger.