codegen/object: Do not generate doc(cfg()) on expressions
codegen/object: Do not generate doc(cfg()) on expressions

Solves warnings of the following kind in gtk-rs:

    warning: unused doc comment
       --> gtk/src/auto/window.rs:309:37
        |
    309 |           #[cfg_attr(feature = "dox", doc(cfg(feature = "v3_20")))]
        |                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^
    310 | /         {
    311 | |             if let Some(ref focus_on_click) = self.focus_on_click {
    312 | |                 properties.push(("focus-on-click", focus_on_click));
    313 | |             }
    314 | |         }
        | |_________- rustdoc does not generate documentation for expressions

Also remove the surrounding expression block and convoluted indentation
logic: if {} is also an expression block that can take attributes.

gtk-rs/gir,GuillaumeGomez/gir,GuillaumeGomez/gir,gtk-rs/gir,gtk-rs/gir,GuillaumeGomez/gir