Add setting to prevent doc auto-hide of trait implementations

src/librustdoc/html/render.rs

@@ -1669,6 +1669,8 @@ impl<'a> Settings<'a> {
             settings: vec![
                 ("item-declarations", "Auto-hide item declarations.", true),
                 ("item-attributes", "Auto-hide item attributes.", true),
+                ("trait-implementations", "Auto-hide trait implementations documentation",
+                 true),
                 ("go-to-only-result", "Directly go to item in search if there is only one result",
                  false),
             ],