Lock down mac os platform type visibility in the rest of GPUI

Add documentation to all platform types

crates/gpui/src/platform/mac/dispatcher.rs

@@ -24,7 +24,7 @@ pub(crate) fn dispatch_get_main_queue() -> dispatch_queue_t {
     unsafe { &_dispatch_main_q as *const _ as dispatch_queue_t }
 }
 
-pub struct MacDispatcher {
+pub(crate) struct MacDispatcher {
     parker: Arc<Mutex<Parker>>,
 }