fixes #19439: add some docs context to migration for .btn-xs and .pag…

…e-header

docs/migration.md

@@ -64,7 +64,7 @@ New to Bootstrap 4 is the Reboot, a new stylesheet that builds on Normalize with
 ### Typography
 
 - Moved all `.text-` utilities to the `_utilities.scss` file.
-- Dropped the `.page-header` class entirely.
+- 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.
 
@@ -96,13 +96,13 @@ New to Bootstrap 4 is the Reboot, a new stylesheet that builds on Normalize with
 ### Buttons
 
 - Renamed `.btn-default` to `.btn-secondary`.
-- Dropped the `.btn-xs` class entirely.
+- Dropped the `.btn-xs` class entirely as `.btn-sm` is proportionally much smaller than v3's.
 - The [stateful button](http://getbootstrap.com/javascript/#buttons-methods) feature of the `button.js` jQuery plugin has been dropped. This includes the `$().button(string)` and `$().button('reset')` methods. We advise using a tiny bit of custom JavaScript instead, which will have the benefit of behaving exactly the way you want it to.
   - Note that the other features of the plugin (button checkboxes, button radios, single-toggle buttons) have been retained in v4.
 
 ### Button group
 
-- Dropped the `.btn-group-xs` class entirely.
+- Dropped the `.btn-group-xs` class entirely given removal of `.btn-xs`.
 
 ### Grid system