Remove dropped btn-xs and btn-group-xs from docs

docs/components/button-group.md

@@ -68,19 +68,12 @@ Instead of applying button sizing classes to every button in a group, just add `
     <button type="button" class="btn btn-secondary">Middle</button>
     <button type="button" class="btn btn-secondary">Right</button>
   </div>
-  <br>
-  <div class="btn-group btn-group-xs" role="group" aria-label="Extra small button group">
-    <button type="button" class="btn btn-secondary">Left</button>
-    <button type="button" class="btn btn-secondary">Middle</button>
-    <button type="button" class="btn btn-secondary">Right</button>
-  </div>
 </div>
 
 {% highlight html %}
 <div class="btn-group btn-group-lg" role="group" aria-label="...">...</div>
 <div class="btn-group" role="group" aria-label="...">...</div>
 <div class="btn-group btn-group-sm" role="group" aria-label="...">...</div>
-<div class="btn-group btn-group-xs" role="group" aria-label="...">...</div>
 {% endhighlight %}
 
 ## Nesting

docs/examples/offcanvas/index.html

@@ -49,7 +49,7 @@
 
         <div class="col-xs-12 col-sm-9">
           <p class="pull-right visible-xs">
-            <button type="button" class="btn btn-primary btn-xs" data-toggle="offcanvas">Toggle nav</button>
+            <button type="button" class="btn btn-primary btn-sm" data-toggle="offcanvas">Toggle nav</button>
           </p>
           <div class="jumbotron">
             <h1>Hello, world!</h1>

docs/migration.md

@@ -77,6 +77,14 @@ New to Bootstrap 4 is the Reboot, a new stylesheet that builds on Normalize with
 
 - Added a new `~480px` grid breakpoint, meaning there are now five total tiers.
 
+### Buttons
+
+- Dropped the `.btn-xs` class entirely.
+
+### Button group
+
+- Dropped the `.btn-group-xs` class entirely.
+
 ### Navs
 
 - Dropped nearly all `>` selectors for simpler styling via un-nested classes.