docs: Add link to usage example to `State` docs (#5165)

core/tauri/src/state.rs

@@ -9,6 +9,8 @@ use crate::{
 use state::Container;
 
 /// A guard for a state value.
+///
+/// See [`Manager::manage`](`crate::Manager::manage`) for usage examples.
 pub struct State<'r, T: Send + Sync + 'static>(&'r T);
 
 impl<'r, T: Send + Sync + 'static> State<'r, T> {