Add @btn-border-radius variables (#16271)

docs/_includes/customizer-variables.html

@@ -400,6 +400,20 @@
     <label for="input-@btn-link-disabled-color">@btn-link-disabled-color</label>
     <input id="input-@btn-link-disabled-color" type="text" value="@gray-light" data-var="@btn-link-disabled-color" class="form-control"/>
   </div>
+  <div class="col-xs-4">
+    <label for="input-@btn-border-radius-base">@btn-border-radius-base</label>
+    <input id="input-@btn-border-radius-base" type="text" aria-describedby="help-block-@btn-border-radius-base" value="@border-radius-base" data-var="@btn-border-radius-base" class="form-control"/>
+    <p id="help-block-@btn-border-radius-base" class="help-block">Allows for customizing button radius independently from global border radius</p>
+  </div>
+  <div class="clearfix"></div>
+  <div class="col-xs-4">
+    <label for="input-@btn-border-radius-large">@btn-border-radius-large</label>
+    <input id="input-@btn-border-radius-large" type="text" value="@border-radius-large" data-var="@btn-border-radius-large" class="form-control"/>
+  </div>
+  <div class="col-xs-4">
+    <label for="input-@btn-border-radius-small">@btn-border-radius-small</label>
+    <input id="input-@btn-border-radius-small" type="text" value="@border-radius-small" data-var="@btn-border-radius-small" class="form-control"/>
+  </div>
 </div>
 <h2 id="forms">Forms</h2>
 <div class="row">