doc: fix <code> inside stability boxes

Fixes: #9714 PR-URL: #9723 Reviewed-By: Jeremiah Senkpiel <[email protected]> Reviewed-By: Luigi Pinca <[email protected]>

Author: silverwind

doc/api_assets/style.css

@@ -362,6 +362,11 @@ tt, code{
 padding:.1em.3em;
 }
 
+.api_stabilitycode{
+background:rgba(0,0,0,.1);
+padding:.1em.3em;
+}
+
 acode{
 color: inherit;
 background: inherit;