Don't position:relative on all pres

We need it for run buttons (rust-lang#44671), but not function defs

src/librustdoc/html/static/rustdoc.css

@@ -1136,8 +1136,13 @@ h3 > .collapse-toggle, h4 > .collapse-toggle {
     font-size: 20px;
 }
 
-pre.rust {
+/* Example code has the "Run" button that
+   needs to be positioned relative to the pre */
+pre.rust.rust-example-rendered {
 	position: relative;
+}
+
+pre.rust {
 	tab-size: 4;
 	-moz-tab-size: 4;
 }