Clarify message `SlotKey` that the `contractID` is never an evm address

[david-bakin-sl]

Problem

Current javadoc at SlotKey.contractID isn't clear (IMO!) that the ContractID must be a contractNum and never an evm_address. IMO "number of the contract" doesn't directly relate to the field contractNum and might be taken to simply mean that the "number" is the "ID". Or so I believe.

Solution

Add a sentence saying this explicitly.

Alternatives

No response

[jsync-swirlds]

Is this issue still relevant after the documentation update?
Here is the current description:
https://github.com/hashgraph/hedera-protobufs/blob/main/services/state/contract/storage_slot.proto#L46