ci: Disallow warnings in rustdoc and test private items (#2341)

Make sure `cargo doc` runs cleanly without any warnings in the CI - some
recently introduced but still allowing a PR to get merged.

In case someone wishes to add docs on private items, make sure those
adhere to the same standards.

src/platform_impl/linux/x11/event_processor.rs

@@ -22,7 +22,7 @@ use crate::{
     event_loop::EventLoopWindowTarget as RootELW,
 };
 
-/// The X11 documentation states: "Keycodes lie in the inclusive range [8,255]".
+/// The X11 documentation states: "Keycodes lie in the inclusive range `[8, 255]`".
 const KEYCODE_OFFSET: u8 = 8;
 
 pub(super) struct EventProcessor<T: 'static> {