doc css: topbar sticks to the top of large windows.

Rationale: for large screens, the convenience of not having to scroll
to the top of the page to do a search outweighs having less vertical
space.
Tested with Chrome, Firefox, Safari with various window and text sizes.

R=adg
CC=gobot, golang-dev
https://golang.org/cl/6493071

doc/style.css

@@ -132,6 +132,24 @@ div#topbar {
 	height: 64px;
 }
 
+/* always show topbar for large screens */
+@media screen and (min-width: 130ex) and (min-height: 900px) {
+       /* 130ex -> wide enough so that title isn't below buttons */
+
+	div#topbar {
+		position: fixed;
+		z-index: 1;
+		top: 0;
+		width: 100%;
+		height: 63px;
+		border-bottom: 1px solid #B0BBC5;
+	}
+
+	div#page {
+       		margin-top: 84px; /* 64+20 to match topbar+H1 */
+	}
+}
+
 body {
 	text-align: center;
 }