Document changes in 6963 (#7441)

* Document 6963

* Minor tweaks

* Minor edit

* Add a note on the changed default value in options.md to keep things in sync

changelog_unreleased/api/pr-6963.md

@@ -0,0 +1,10 @@
+#### Change default value for `trailingComma` to `es5` ([#6963](https://github.com/prettier/prettier/pull/6963) by [@fisker](https://github.com/fisker))
+
+Before version 2.0, Prettier was avoiding trailing commas by default where possible.
+This made the resulting JavaScript compatible with now very old environments such as IE8, but implied [some missed opportunities](https://medium.com/@nikgraf/why-you-should-enforce-dangling-commas-for-multiline-statements-d034c98e36f8).
+
+Prettier has included an [option to configure trailing commas](https://prettier.io/docs/en/options.html#trailing-commas) since its early days, and an initiative to change the default value has been out there [for over three years](https://github.com/prettier/prettier/issues/68).
+Finally, the default value becomes `es5` instead of `none` in Prettier v2.0.
+
+If the old behavior is still preferred, please configure Prettier with `{ "trailingComma": "none" }`.
+There is a possibility that the default value will change to `always` (meaning even more trailing commas) in a future major version of Prettier as the JavaScript ecosystem further matures.

docs/options.md

@@ -93,6 +93,8 @@ Use single quotes instead of double quotes in JSX.
 
 ## Trailing Commas
 
+_Default value changed from `none` to `es5` in v2.0.0_
+
 Print trailing commas wherever possible when multi-line. (A single-line array, for example, never gets trailing commas.)
 
 Valid options: