bpo-34162: Update idlelib/NEWS.txt to 2018-12-20. (#11255)

bpo-34162: Update idlelib/NEWS.txt to 2018-12-20. (#11255) · python/cpython@87ec110

Original file line number	Diff line number	Diff line change
`@@ -3,6 +3,9 @@ Released on 2019-10-20?`
`3`	`3`	`======================================`
`4`	`4`
`5`	`5`
	`6`	`+bpo-35521: Document the editor code context feature.`
	`7`	`+Add some internal references within the IDLE doc.`
	`8`	`+`
`6`	`9`	`bpo-34864: When starting IDLE on MacOS, warn if the system setting`
`7`	`10`	`"Prefer tabs when opening documents" is "Always". As previous`
`8`	`11`	`documented for this issue, running IDLE with this setting causes`