Clean up GetStorage

[haerdib]

The current GetStorage ( https://github.com/scs/substrate-api-client/blob/7e5962bd2c9149f990b34aa25ad7bc7f6f92c868/src/api/rpc_api/state.rs) uses some outdated namings.
E.g.

substrate-api-client/src/api/rpc_api/state.rs

Lines 59 to 63 in 7e5962b

	fn get_storage_by_key_hash<V: Decode>(
	&self,
	key: StorageKey,
	at_block: Option<Hash>,
	) -> Result<Option<V>>;

does not get the storage by Key Hash but by StorageKey. That should be clarified.

Additionally, some explanatory comments would be nice. Check out Substrate's comments for reference: https://github.com/paritytech/substrate/blob/7d233c2446b5a60662400a0a4bcfb78bb3b79ff7/client/rpc-api/src/state/mod.rs