rustdoc: add CSS margin between impl docblock and its items

src/librustdoc/html/static/css/rustdoc.css

@@ -1872,6 +1872,7 @@ in storage.js
 }
 
 .variants > .docblock,
+.implementors-toggle > .docblock,
 .impl-items > .rustdoc-toggle[open]:not(:last-child),
 .methods > .rustdoc-toggle[open]:not(:last-child),
 .implementors-toggle[open]:not(:last-child) {

src/test/rustdoc-gui/impl-doc.goml

@@ -0,0 +1,9 @@
+// A docblock on an impl must have a margin to separate it from the contents.
+goto: "file://" + |DOC_PATH| + "/test_docs/struct.TypeWithImplDoc.html"
+
+// The text is about 24px tall, so if there's a margin, then their position will be >24px apart
+compare-elements-position-near-false: (
+	"#implementations-list > .implementors-toggle > .docblock > p",
+	"#implementations-list > .implementors-toggle > .impl-items",
+	{"y": 24}
+)

src/test/rustdoc-gui/src/test_docs/lib.rs

@@ -442,3 +442,11 @@ fn function() {}
         fn function2() {}
     }
 }
+
+pub struct TypeWithImplDoc;
+
+/// impl doc
+impl TypeWithImplDoc {
+    /// fn doc
+    pub fn test_fn() {}
+}