monero-wallet-rpc documentation additions

Hi, I found a few more items to document for monero-wallet-rpc:

1. document get_transfers account_index and subaddr_indices defaults (they both default to 0)
2. tx_size is listed as a return field on incoming_transfers but that does not return as far as I can see
3. rescan_blockchain rebuilds the wallet cache which destroyed local data like outgoing destination addresses. I think that should be documented.