Mention .list-inline-item change in the migration guide

Fixes twbs#18813
Closes twbs#20187

[skip sauce]

docs/migration.md

@@ -67,6 +67,7 @@ New to Bootstrap 4 is the Reboot, a new stylesheet that builds on Normalize with
 - Dropped `.page-header` as, aside from the border, all it's styles can be applied via utilities.
 - `.dl-horizontal` has been dropped. Instead, use `.row` on `<dl>` and use grid column classes (or mixins) on its `<dt>` and `<dd>` children.
 - Custom `<blockquote>` styling has moved to classes—`.blockquote` and the `.blockquote-reverse` modifier.
+- `.list-inline` now requires that its children list items have the new `.list-inline-item` class applied to them.
 
 ### Images