Clarify use of `prefunded` parameter to callbacks

[0xJem]

Inspired by sherlock-audit/2024-03-axis-finance-judging#16

We use the prefunded parameter to indicate if the callback should send tokens, when that's actually determined by the send base tokens flag.

e.g. Not prefunded, send base tokens true, prefunded true: https://github.com/sherlock-audit/2024-03-axis-finance/blob/cadf331f12b485bac184111cdc9ba1344d9fbf01/moonraker/src/AuctionHouse.sol#L265

Not prefunded, send base tokens false, prefunded true: https://github.com/sherlock-audit/2024-03-axis-finance/blob/cadf331f12b485bac184111cdc9ba1344d9fbf01/moonraker/src/AuctionHouse.sol#L295

We should clarify what it actually means