Tweak doc css to avoid overlapping with search box

Make the title lower, so it no longer overlaps with the search box.

Before and after screenshots: http://imgur.com/a/s7CO9

Change-Id: Ibfe062b34dc2af665aad239eefd498407cc34915

Documentation/doc.css.in

@@ -17,6 +17,10 @@ h1, h2, h3 {
   border-bottom: 2px solid silver;
 }
 
+h1 {
+  margin-top: 1.5em;
+}
+
 p {
   margin: 0.5em 0 0.5em 0;
 }