Added some docs comments to better identify fields that are just named `id`

[maoueh]

And to better spot where transaction hash can be found.

[maoueh]

I need to re-write the comment removing mention of transactionHash as I mixed up Protobuf fields when I checked, so it's totally false, id are all receiptId.

[azf20]

@maoueh checking the status of this, my understanding was that it's not possible to get the tx hash for the receipt at this time, but your original comment suggested it was?

[maoueh]

@azf20 That’s correct, I thought one of the id was the transaction hash/id, but I was wrong and all id fields are actually referring to the receipt id.

So, there is no way to obtain today the transaction hash (some changes in graph-node is required to support that, probably passing more context to the receipt handler).