rustdoc: add missing margin to no-docblock trait items

Fixes another regression caused by 8846c08,
this time fixing the appearance of methods that have no docblock (we didn't
notice this one because libstd docs *always* have docblocks).

See how it looks at https://doc.rust-lang.org/nightly/nightly-rustc/rustdoc/clean/types/trait.AttributesExt.html

Author: notriddle

src/librustdoc/html/render/print_item.rs

@@ -718,7 +718,7 @@ fn item_trait(w: &mut Buffer, cx: &mut Context<'_>, it: &clean::Item, t: &clean:
         if toggled {
             write!(w, "<details class=\"rustdoc-toggle method-toggle\" open><summary>");
         }
-        write!(w, "<div id=\"{}\" class=\"method has-srclink\">", id);
+        write!(w, "<section id=\"{}\" class=\"method has-srclink\">", id);
         render_rightside(w, cx, m, t, RenderMode::Normal);
         write!(w, "<h4 class=\"code-header\">");
         render_assoc_item(
@@ -730,7 +730,7 @@ fn item_trait(w: &mut Buffer, cx: &mut Context<'_>, it: &clean::Item, t: &clean:
             RenderMode::Normal,
         );
         w.write_str("</h4>");
-        w.write_str("</div>");
+        w.write_str("</section>");
         if toggled {
             write!(w, "</summary>");
             w.push_buffer(content);

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

@@ -2010,7 +2010,8 @@ in storage.js plus the media query with (min-width: 701px)
 .impl,
 #implementors-list > .docblock,
 .impl-items > section,
- {
+.methods > section
+{
 	margin-bottom: 0.75em;
 }