[Docs] Use <dl> for definition lists of variables

Hopefully making them more semantic and easier to read,
though it is raw HTML so it is slightly more work to maintain.

Also made minor revisions to some of the variable descriptions
to be more informative, e.g. `:monthname` in permalinks use
full English names ("January" etc.)

docs/static/css/style.css

@@ -500,3 +500,23 @@ td.purpose-title {
 td.purpose-description {
   border-left: 0;
 }
+
+/* For definitions of variables */
+
+dl {
+    margin: 1em;
+    border-bottom: 1px solid #ccc;
+}
+
+dt {
+    float: left;
+    clear: left;
+    width: 9.5em;
+    margin: 0.125em;
+    padding: 2px 4px;
+}
+
+dd {
+    padding: 0.2em 0 0.2em 10em;
+    border-top: 1px solid #ccc;
+}