Fix comment on NoMangle

Signed-off-by: Jonathan Brouwer <jonathantbrouwer@gmail.com>

src/librustdoc/clean/types.rs

@@ -753,7 +753,8 @@ impl Item {
             .other_attrs
             .iter()
             .filter_map(|attr| {
-                // NoMangle is special-cased because cargo-semver-checks uses it
+                // NoMangle is special cased, as it appears in HTML output, and we want to show it in source form, not HIR printing.
+                // It is also used by cargo-semver-checks.
                 if matches!(attr, hir::Attribute::Parsed(AttributeKind::NoMangle(..))) {
                     Some("#[no_mangle]".to_string())
                 } else if is_json {