rollup merge of #20092: barosl/rustdoc-line-number-clickable

While talking on IRC, someone wanted to post a link to the Rust source code, but while the lines of the rendered source code do have anchors (`<span id="[line number]">`), there is no convenient way to make links as they are not clickable. This PR makes them clickable.

Also, a minor fix of the FAQ is included.

src/librustdoc/html/render.rs

@@ -2253,9 +2253,9 @@ impl<'a> fmt::Show for Source<'a> {
             cols += 1;
             tmp /= 10;
         }
-        try!(write!(fmt, "<pre class='line-numbers'>"));
+        try!(write!(fmt, "<pre class=\"line-numbers\">"));
         for i in range(1, lines + 1) {
-            try!(write!(fmt, "<span id='{0}'>{0:1$}</span>\n", i, cols));
+            try!(write!(fmt, "<span id=\"{0}\">{0:1$}</span>\n", i, cols));
         }
         try!(write!(fmt, "</pre>"));
         try!(write!(fmt, "{}", highlight::highlight(s.as_slice(), None, None)));