Inverse card improvements (#21242)

* Add a color to .card-inverse to account for non .card- prefixed classes

* reword the docs to better articulate what the class does

scss/mixins/_cards.scss

@@ -20,6 +20,8 @@
 //
 
 @mixin card-inverse {
+  color: rgba(255,255,255,.65);
+
   .card-header,
   .card-footer {
     background-color: transparent;