Add some visually difference on the hoverxref links

Author: humitos

hoverxref/_static/css/tooltipster.custom.css

@@ -1,3 +1,8 @@
+.hoverxref {
+    border-bottom: 1px dotted;
+    border-color: gray;
+}
+
 .tooltipster-box {
     max-height: 600px;
 }