chore: fix JsonStorageKey comment (#2788)

crates/primitives/src/serde_helper/storage_key.rs

@@ -2,8 +2,25 @@ use crate::{H256, U256};
 use serde::{Deserialize, Serialize};
 use std::fmt::Write;
 
-/// A storage key type that can be serialized to and from a hex string up to 40 characters. Used
-/// for `eth_getStorageAt` and `eth_getProof` RPCs.
+/// A storage key type that can be serialized to and from a hex string up to 32 bytes. Used for
+/// `eth_getStorageAt` and `eth_getProof` RPCs.
+///
+/// This is a wrapper type meant to mirror geth's serialization and deserialization behavior for
+/// storage keys.
+///
+/// In `eth_getStorageAt`, this is used for deserialization of the `index` field. Internally, the
+/// index is a [H256], but in `eth_getStorageAt` requests, its serialization can be _up to_ 32
+/// bytes. To support this, the storage key is deserialized first as a U256, and converted to a
+/// H256 for use internally.
+///
+/// `eth_getProof` also takes storage keys up to 32 bytes as input, so the `keys` field is
+/// similarly deserialized. However, geth populates the storage proof `key` fields in the response
+/// by mirroring the `key` field used in the input.
+///  * See how `storageKey`s (the input) are populated in the `StorageResult` (the output):
+///  <https://github.com/ethereum/go-ethereum/blob/00a73fbcce3250b87fc4160f3deddc44390848f4/internal/ethapi/api.go#L658-L690>
+///
+/// The contained [H256] and From implementation for String are used to preserve the input and
+/// implement this behavior from geth.
 #[derive(Clone, Debug, Default, Eq, PartialEq, Serialize, Deserialize)]
 #[serde(from = "U256", into = "String")]
 pub struct JsonStorageKey(pub H256);