FIP-0092: clarify wording in gas calculation section

[rvagg]

NI-PoRep, already active in the network. While revisiting the gas calculations I found this particularly impenetrable when viewed from a per-sector view. This is my attempt to clear up the confusion for the future visitor who isn't as privy to the snark/circuit/proof/sector nuances involved.

Preview rendered version of this here.

[lucaniz]

Same here :)