rustdoc: Restore underline text decoration on hover for FQN in header

This causes the components of FQN's to behave similarly to other links
in the contents of rustdoc-styled pages.

I (and I hope others at least in part) have found the prior design to be
somewhat confusing, as it is not clear (upon hovering) that the various
parts of the FQN are actually links that the user can navigate to.

In short, this patch makes links in the FQN have an underline when the
user hovers over them, more clearly indicating that they can be used for
navigation.

Signed-off-by: Kristofer Rye <[email protected]>

src/librustdoc/html/static/rustdoc.css

@@ -100,6 +100,9 @@ h1.fqn {
 	border-bottom: 1px dashed;
 	margin-top: 0;
 }
+h1.fqn > .in-band > a:hover {
+	text-decoration: underline;
+}
 h2, h3:not(.impl):not(.method):not(.type):not(.tymethod), h4:not(.method):not(.type):not(.tymethod):not(.associatedconstant) {
 	border-bottom: 1px solid;
 }