Update cache document (#669)

crates/cache/src/lib.rs

@@ -231,7 +231,22 @@ impl CachedEntry {
     }
 }
 
-/// A constructed via `salvo_cache::Cache::builder()`.
+/// Cache middleware.
+///
+/// # Example
+///
+/// ```
+/// use std::time::Duration;
+///
+/// use salvo_core::Router;
+/// use salvo_cache::{Cache, MokaStore, RequestIssuer};
+///
+/// let cache = Cache::new(
+///     MokaStore::builder().time_to_live(Duration::from_secs(60)).build(),
+///     RequestIssuer::default(),
+/// );
+/// let router = Router::new().hoop(cache);
+/// ```
 #[non_exhaustive]
 pub struct Cache<S, I> {
     /// Cache store.