PRFC1: Clarify what happens when someone `transfer_from`s themselves

[lyulka]

i.e., what happens when the from_address argument to transfer_from is equal to txn.signer.

Should such a call be equivalent to:

1. A non-delegated transfer?
2. Or, should the method still check whether I have "given myself an allowance"? (I assume that by default, our "self-allowance" should be 0).

[manngayin612]

From my understanding, this situation is equivalent to (2.).
There's is no check whether txn.signer == from_address in transfer_from() now.
ERC20 doesn't handle this special case too, it also just treat it as "i have given myself an allowance",