godoc: added alt and title attributes to the "pop out" button

R=adg
CC=golang-dev
https://golang.org/cl/5777071

doc/root.html

@@ -5,7 +5,7 @@
 <div class="left">
 
 <div id="learn">
-<img class="icon share" src="/doc/share.png">
+<img class="icon share" src="/doc/share.png" alt="View full screen" title="View full screen">
 <div class="rootHeading">Try Go</div>
 <div class="input">
 <textarea spellcheck="false" class="code">// You can edit this code!