Add cautions for cache_set

crates/pink/pink-extension/src/chain_extension.rs

@@ -257,6 +257,14 @@ pub trait PinkExt {
     ///
     /// * `Result<(), StorageQuotaExceeded>` - `Ok(())` or `Err(StorageQuotaExceeded)` if the storage quota is exceeded.
     ///
+    /// # Caution
+    ///
+    /// The cache is not guaranteed to be persistent. It may be cleared at any time due
+    /// to various reasons:
+    /// - The cached item is expired.
+    /// - The entire cache in pRuntime is full and a new value needs to be stored.
+    /// - The worker is restarted.
+    ///
     /// # Example
     ///
     /// ```