Auto merge of #35667 - ollie27:rustdoc_opaque_structs, r=steveklabnik

rustdoc: Don't add extra newlines for fully opaque structs

Changes the definition for braced structs with only private or hidden fields to save space on the page.

Before:
```
pub struct Vec<T> {
    // some fields omitted
}
```
After:
```
pub struct Vec<T> { /* fields omitted */ }
```

This also cleans up empty braced structs.

Before:
```
pub struct Foo {
}
```
After:
```
pub struct Foo {}
```

[before](https://doc.rust-lang.org/nightly/std/vec/struct.Vec.html) [after](https://ollie27.github.io/rust_doc_test/std/vec/struct.Vec.html)

cc #34713

src/test/rustdoc/structfields.rs

@@ -48,3 +48,13 @@ pub enum Qux {
         // @has - //pre "// some fields omitted"
     },
 }
+
+// @has structfields/struct.Baz.html //pre "pub struct Baz { /* fields omitted */ }"
+pub struct Baz {
+    x: u8,
+    #[doc(hidden)]
+    pub y: u8,
+}
+
+// @has structfields/struct.Quux.html //pre "pub struct Quux {}"
+pub struct Quux {}