chore: add subkey docs to StorageEntry (#4016)

crates/primitives/src/storage.rs

@@ -3,6 +3,8 @@ use reth_codecs::{derive_arbitrary, Compact};
 use serde::{Deserialize, Serialize};
 
 /// Account storage entry.
+///
+/// `key` is the subkey when used as a value in the `StorageChangeSet` table.
 #[derive_arbitrary(compact)]
 #[derive(Debug, Default, Copy, Clone, PartialEq, Eq, Serialize, Deserialize, PartialOrd, Ord)]
 pub struct StorageEntry {