Adjust CSS to have tooltips in the H2 headings in the docs

There's been at least one complaint about a stray # that appears in the
manual.  Add a tooltip to this to make it more clear why this appears
and what it's for.

Reported-by: Muhammad Ikram <[email protected]>
Reviwed-by: Daniel Gustafsson <[email protected]>
Discussion: https://postgr.es/m/CAGeimVqnkm7X-cXNNXjdGL2u5N29RiFtcpUzPuW1-VYACe6m+Q@mail.gmail.com

Author: david-rowley

media/css/main.css

@@ -1183,6 +1183,20 @@ code,
 #docContent a.id_link {
   color: inherit;
   visibility: hidden;
+  text-decoration: none;
+}
+
+#docContent a.id_link:hover:after {
+  content: "Permalink to this heading";
+  font-size: 10px;
+  font-weight: normal;
+  text-decoration: none;
+  border: 1px solid #444444;
+  text-align: center;
+  border-radius: 5px 5px 5px 5px;
+  padding: 5px 5px 5px 5px;
+  position: relative;
+  top: 20px;
 }
 
 #docContent *:hover > a.id_link {