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/macos/observer.rs

@@ -99,7 +99,8 @@ pub type CFRunLoopTimerCallBack = extern "C" fn(timer: CFRunLoopTimerRef, info:
 pub enum CFRunLoopTimerContext {}
 
 /// This mirrors the struct with the same name from Core Foundation.
-/// https://developer.apple.com/documentation/corefoundation/cfrunloopobservercontext?language=objc
+///
+/// <https://developer.apple.com/documentation/corefoundation/cfrunloopobservercontext?language=objc>
 #[allow(non_snake_case)]
 #[repr(C)]
 pub struct CFRunLoopObserverContext {