Add new table functionality to table migration section

Calls out the new `.table-inverse`, `.table-reflow`, `.thead-default` and `.thead-inverse`

docs/migration.md

@@ -60,6 +60,8 @@ New to Bootstrap 4 is the Reboot, a new stylesheet that builds on Normalize with
 - Responsive tables no longer require a wrapping element. Instead, just put the `.table-responsive` right on the `<table>`.
 - Renamed `.table-condensed` to `.table-sm` for consistency.
 - Added a new `.table-inverse` option.
+- Added a new `.table-reflow` option.
+- Added table header modifers: `.thead-default` and `.thead-inverse`
 
 ### Forms