Rollup merge of rust-lang#72421 - GuillaumeGomez:fix-impl-hover-ancho…

…r, r=kinnison

Fix anchor display when hovering impl

A little gif for the fixed behaviour:

![hover-anchor](https://user-images.githubusercontent.com/3050060/82549808-cfdf4080-9b5d-11ea-9495-2b1d90b2a791.gif)

r? @kinnison

src/librustdoc/html/static/rustdoc.css

@@ -625,7 +625,7 @@ a {
 	display: initial;
 }
 
-.in-band:hover > .anchor {
+.in-band:hover > .anchor, .impl:hover > .anchor {
 	display: inline-block;
 	position: absolute;
 }